Notes Flashcards
Flow Rate
The number of traffic units per unit time. For road traffic, the units are vehicles per hour (veh/hr) or day (veh/day), sometimes with the type of vehicle more precisely defined, e.g. passenger car units per hour, or cyclists per hour.
Traffic flow rates or volumes are used to establish:
• Relative importance and role of a road in a traffic system;
• Variations in the levels of traffic flow over time;
• Extent of the use of a facility in terms of its capacity to carry traffic;
• Distribution of travel demand in a network; and
• Coordination of traffic signals, etc.
Link Count
The number of vehicles passing an observation point along a road link over a given period. The count may be bi-directional or may be split into separate counts for the two directions of flow.
Turning Movement Count
The number of vehicles observed to make a particular turning movement at an intersection over a specified period.
AADT
Annual Average Daily Traffic: the total volume of traffic passing a roadside observation location over the period of a calendar year, divided by the number of days in that year. AADT is usually expressed in terms of actual vehicles per day.
ADT
Average Daily Traffic or Count: A traffic count averaged over a period less than a year, such as a month, week or a few days.
HHV
Highest Hourly Volume: The highest hourly volume of any continuing 60-min period over a whole year. HHV is usually rated in terms of an ‘nth’ highest hour volume, meaning the hourly traffic volume (veh/hr) exceeded in only (n) hours of a year. This concept is chosen because it is uneconomic to design a facility to meet the highest traffic flow rate.
DHV
Design Hour Volume: The traffic flow rate chosen as the design traffic load for a facility over its design life. Common practice is to choose an ‘nth’ HHV as the design volume, with the 30th highest hourly volume (30HV) often used in rural environment and the 80HV in an urban area.
PHV
Peak Hour Volume: the maximum traffic count observed in any 60-min interval during a day.
PHF
Peak Hour Factor: The ratio of total hourly volume to the maximum flow rate over a specific time period within that hour (e.g. 15mins).
VKT
Provides a measure of the total level of usage of a road or road system. It is important in economic evaluation and also as an exposure measure in road crash studies.
Manual Counts
Are usually carried out at intersections where turning movement volumes are required or at sites where detailed classification data are needed, such as the number of vehicles making particular turns, types of vehicles, etc. Manual counting is much more expensive than automatic counting and is generally used for short period studies only. Video cameras can also be used and manually log data later.
Automatic Counters
Normally used for recording 24-hour counts and the hourly, daily, or seasonal variations in traffic volumes. The equipment normally consists of a data logger and an axle or vehicle sensor.
Axle Counts
Axles are generally detected by a pneumatic rubber tube stretched across the road surface. The pulse generated in the tube when an axle crosses it closes the contact at the connect air-switch, and so the axle is registered. Special electrical cables such as ‘triboelectric’ (friction sensitive) and ‘peizoelectric’ (pressure sensitive) cables may also be used as axle detectors. A further type of axle detector is the ‘tape switch’ or ‘treadle-switch’, which consists of two trips of metal pushed together to complete an electric circuit by the passage of an axle.
Automatic counting equipment that use pneumatic tubes and other axle detectors either provide counts of ‘axle pairs’ from one detector, or classify the vehicles according to axle spacing’s using two detectors.
Vehicle Counts
An alternative form of traffic detection is to register the passage and/or presence of a vehicle. The most used vehicle presence detector is the inductive loop. Other technologies include microwave or radar scanning, infrared, acoustic, magnetic and video imaging devices.
Inductive Loop Sensor
By far the most used vehicle count technology. It consists of several loops of wire embedded in the pavement, or attached to the road surface, as a temporary detector.
An alternating current is passed through the inductive loop. When a mass of metal such as a vehicle chassis and an engine passes through the electromagnetic field of the loop, the inductance of the loop changes. These changes are used to indicate the passage or presence of a vehicle.
For accurate counting, care needs to be taken in selecting the size, shape and positioning of the loop within the traffic lanes. Over-counting can occur when loops in adjacent lanes are too close and the same vehicle is detected in both lanes. On the other hand, motorcyclists or small vehicles, straddling both lanes may be missed by loops placed too far apart.
Induction loops for counting are often a square of 2m x 2m, and a pair of these loops at a known distance (about 4m) apart is usually used for speed measurement and vehicle classification.
The inductive loop detector is used extensively for automatic traffic counting, traffic surveillance and traffic signal control.
Daily Variations
Hour by hour changes in levels of traffic demand. Distinct peaks and directional differences in flows, may be observed.
Weekly Variations
Between weekend and weekday
Seasonal Variations
Urban roads generally show small variations, whereas rural roads may show significant changes.
Trend Effects
Arise from changes in the general levels of traffic activity at a site over an extended period, as a reflection of changes in land use, population and economic activity in a region.
Estimation of Design Hourly Volume (DHV)
When designing a road, a balance must be made between the investment cost and the level of service provided. The objective of the designer is to achieve the desired level of service at acceptable costs. Traffic demand can vary widely in a day and throughout a year, and it would be uneconomical to design a road for the maximum hourly volume that could be expected.
Estimation of Vehicle Kilometres of Travel (VKT)
The total daily travel on all segments in a road network is the sum of the product of AADT on each segment and the segment length.
Time Mean Speed
The arithmetic mean of the measured spot speeds of all vehicles passing a fixed roadside point during a given time interval
Space Mean Speed
The arithmetic mean of the measured speeds of all vehicles in the stream which are within a specified length of roadway at a given instant of time.
Microwave Radar Guns
Used for speed measurements make use of the Doppler effect. A microwave beam is sent to the target vehicle, which reflects back a signal to the receiver in the radar gun. The moving vehicle affects the frequency of the returned signal. The shift in the frequencies between the emitted and received microwave signals is called the Doppler effect. By measuring the amount of frequency shift and the duration of the time interval, the speed of the targeted vehicle can be determined. A microwave radar gun has a wide cone of detection, which is about 70 m at a range of 300 m
Laser infrared Gun
Uses the higher optical frequency and has a small detection cone of about 1 m in diameter at a distance of 300 m between the laser gun and the targeted vehicle. The equipment employs a direct method because it relies on the measurement of the round-trip time of the infrared light beam to reach a vehicle and be reflected back. The gun can accurately count the number of nanoseconds the light takes for the round trip, and making use of the speed of light at 300,000 km/s, several samples of the distance are obtained in a fraction of a second. The changes in distance and hence the velocity as the targeted vehicle moves can be measured accurately.
Video
Can be used to determine vehicle speeds and is becoming increasingly cheaper to use and operate. The general method involves recording the distance moved by a vehicle in a short period (perhaps a couple of frames), then computing the speed. Manual data extraction from a video recording is time-consuming, tedious and expensive, however, automatic data extraction procedures is a cost-effective alternative. Video imaging techniques are used to extract speed data from fixed position cameras. The operator defines virtual loops on a carriageway and superimposes them on the video frames captured by the camera. The luminance of the marked area changes as one or more vehicles travel over the area. These changes are detected and provide information such as speed, volume, headway and occupancy. The accuracy of speed measurement using video is affected by shadows of adjacent lane traffic and weather conditions, and is usually less than that obtained using inductive loop sensors.
GPS Receiver
Whether in point processing or differential mode, keeps track of its position and time. From time and position, it can also calculate its speed when in motion. The speed values can be useful as an indicator of the road congestion condition. In Singapore, such data are transmitted from GPS-equipped taxis to a traffic control centre. The data are processed and then made available in real-time on the Internet as congestion indicators.
Free Flow Time
The time required by an unimpeded vehicle to traverse the survey section
Free Flow Speed
The length of the survey section divided by the free flow time
Travel Time
The actual (observed) time taken to traverse the test section
Delay
The difference between the travel time and the free flow travel time
Stopped Time
The period for which a vehicle is stationary while in the survey section
Running Time
The period of time for which a vehicle is in motion while in the survey section; (total) travel time is then the sum of stopped time and running time
Running Speed
The total sectional distance divided by the running time; sometimes running speed is used as an estimate of free flow speed.
Moving Observer Methods
Moving observer methods enable estimates mean travel times to be made fairly easily and quickly. The basic resources required are a test vehicle, two people (driver and observer), and a data recording system (manual or automated). An instrumented vehicle is one equipped with an ‘in-built’ computer data recording system.
Floating Car Method
The floating car method measures the mean travel time along a test section. A test vehicle attempts to simulate an ‘average’ vehicle in the traffic stream, by noting the number of vehicles that overtake the test car and the number of vehicles that the test car overtakes – and the test vehicle is said to be ‘floating’ in the traffic if the two numbers are the same. The floating car method is appropriate for travel time surveys on long and complex routes. The main advantage of the method is that data can be collected easily and quickly. The main disadvantage is that it is difficult and expensive to collect large samples of data, as the test vehicle has to traverse the route and then return to the starting point for the next run. This all takes time, and in a dynamic environment where travel conditions (eg flow rates and traffic control systems) are changing rapidly, repeated runs may form biased samples of travel times from any one set of conditions. Further problems exist for the technique on arterial roads with significant levels of platooning, in which the test vehicle may have great difficulty in floating in the stream.
Chase Car Method
Here the survey vehicle follows a randomly selected vehicle in the traffic stream, copying as closely as possible the manoeuvres of the chased vehicle. For travel time studies it is necessary to keep track of the time performance of the vehicle (eg section travel times, time stopped). Care is needed to ensure that vehicles are selected at random, or strictly according to a predetermined strategy (eg by tracking a certain number of particular vehicle types) – otherwise significant biases may result. One obvious problem with the method is that the chased vehicle may leave the survey route at any time. The decision then has to be made as to whether the survey vehicle aborts the run and returns to pick up a new vehicle; continues and picks up a nearby vehicle to follow; or ‘floats’ in the traffic until the end of the test section.
Number Plate Survey Method
These surveys may also be used to collect data on the distribution of travel times in a section of the road network. The survey operates by having observers positioned at selected points on the road: they record the number plates (or partial number plates) and arrival times of a sample of the vehicles that pass them. When the data are consolidated, number plates can be matched between upstream and downstream locations and travel times computed. The suggested practical maximum rate for conventional data recording methods is 600 recordings per hour. The selection of appropriate recording equipment and proper training of observers is essential. A problem is vehicles not completing the journey so more data has to be collected to enable appropriate numbers of matching number plates.
Input-Output Survey
The technique is based on the idea that the difference between the means of two sets of observations is equal to the mean of the differences of the two sets. Input-output surveys find the mean arrival time and the mean departure time of the traffic stream in the test section, and calculate the mean travel time by subtracting mean departure time from mean arrival time. The data collected at each station are the number of arrivals in successive time intervals. The shorter the time intervals, the more accurate is the estimate, but also the higher the workloads imposed on field staff and consequently, the less accurate the observations. Input-output analysis is best suited to closed systems, such as motorways, in which vehicles entering the survey zone can only leave it via the observation points. Another application for the technique is in recording the duration of stay of vehicles in an off-street car park.
Path Trace Survey
This method is capable of providing accurate information about traffic movements and travel times in a restricted area. Examples are direct observation, video recordings or even vehicle tagging using GPS and other devices. The travel time and detailed path of a vehicle can be determined. The sampling of vehicles may be a problem. A suitable vantage point is also needed for direct observation or video recording.
Queuing Survey
Measurement of queue lengths involves an observer recording the number of stationary vehicles at a particular point in time. This can be done by physically counting the vehicles, or by placing marks along the road length to indicate the number of vehicles that would be in a queue of a given physical length. Video cameras can be used to record the queue lengths for subsequent analysis manually, or automatically employing digital imaging technologies.
Stopped Delay
The delay experienced by vehicles that have actually stopped. It is one component of overall delay, which also includes the delay resulting from a vehicle having to slow down because of interactions with other vehicles. The measurement of intersection delay using the point-sample method requires the identification of two locations, which could be the stopline and a point further back than the tail end of any expected queue. The sampling can be at small, regular intervals for stopped delay or at specific time points of a signal cycle for overall delay. Stopped time delay can be collected by a manual method, which separately records stopped vehicles in small intervals, as well as all previously stopped and departed vehicles.
Road Hierarchy
Mobility vs Access – as the roads increase with size the access decreases by the mobility increases.
- Local Network
- Collectors
- Arterials
- Motorways
Uninterrupted Traffic Flow
Occurs in a traffic stream that is not delayed or interfered with by factors external to the traffic stream itself e.g. motorways.
Interrupted Traffic Flow
Flow which is regulated e.g. by a traffic signal. Under interrupted flow conditions, vehicle interactions and vehicle-roadway interactions play a secondary role in defining the traffic flow.
Flow (Volume, q)
number of vehicles per unit time passing a given point (veh/s).
q=n/t
Where:
q = traffic flow in vehicles per unit time;
n = number of vehicles passing some designated roadway point during time t; and
t = duration of time interval
Headway
Time between the passage of the front bumpers of successive vehicles, at some designated highway point.
q= 1/h
h ̅= ∑hi/n
Where:
hi= time headway of the ith vehicle (the time that has transpired between the arrival of vehicle i and i-1);
h ̅ = is the average time headway, in unit time per vehicle
n = number of measured vehicle time headways at some designated roadway point.
Speed
Average traffic speed is defined in two ways
Time-mean speed (u ̅t):
(u_t ) ̅= (∑_(i=1)^n▒u_i )/n
Where:
(u_t ) ̅ = Time-mean speed in unit distance per unit time,
u_i = Spot speed of the ith vehicle, and
n = number of measured vehicle spot speeds.
Space-mean speed ((u_s ) ̅): The Harmonic Speed – this method is more useful in the context of traffic analysis and is determined on the basis of the time necessary for a vehicle to travel some known length of roadway. (u_s ) ̅= 1/(1/n ∑_(i=1)^n▒[1/(l⁄t_i )] ) Where: (u_s ) ̅ = space-mean speed in unit distance per unit time, ti = travel time of the ith vehicle between two designated points, n = number of measured vehicles
Density
Density is the number of vehicles present within a unit length of lane or road at a given instant of time (veh/km).
k=n/l
Where:
k = traffic density in vehicles per unit distance;
n = number of vehicles occupying some length of roadway at some specified time, and
l = length of roadway
Spacing
The distance between the passage of the front bumpers of successive vehicles, at some designated highway point.
k=l/s ̅
s ̅= ∑_(i=1)^n▒〖s_i/n〗
Where:
si = distance of the ith vehicle (the distance that is between the vehicle i and i-1);
s ̅ = is the average distance, in unit distance per vehicle
n = number of measured vehicle distance headways at some designated roadway point.
Traffic Flow Relationships
q=k×u Where: q = volume or flow (veh/hour) k = density (veh/km) u = space mean speed (km/h) (us)
q=1/h k=1/s Where: q = volume or flow (veh/hr) h = headway (s/veh) k = density (veh/km) s = spacing (m/veh)
Speed Density Model
u= u(1- (k/k_j )) Where: u = space-mean speed in km/h uf = free-flow speed in km/h k = density in veh/km kj = jam density in veh/km
Flow Density Model
q= uf (k- (k^2/kj ))
Poisson Distribution
Models that account for the nonuniformity of flow are derived by assuming that the pattern of vehicle arrivals (at a specified point) corresponds to some random process. P(n) the probability of n occurrences of an event in a situation for which the expected number of occurrences is m. Where m is continuous and n is discrete.
P(n)= (〖(qt)〗^n e^(-qt))/n!
Where:
P(n) = probability of having n vehicles arrive in time t,
t= duration of the time interval over which vehicles are counted,
q = average vehicle flow or arrival rate in vehicles per unit time.
Limitations of Poisson Distribution
• Poisson most realistic in lightly congested traffic conditions
• Poisson appropriate if mean of period of observations approximately equals the variance
o If the variance is significantly greater than the mean, the data are said to be overdispersed
o If the variance is significantly less than the mean, the data are said to be underdispersed.
Negative Exponential Distribution
The probability of a vehicle headway, h, being greater than or equal to a time interval of length t is equivalent to the probability of having no vehicles arrive in the time interval t (P(0)).
Characteristics of a Queue
- Arrival pattern, arrival rate
- Service pattern, service rate
- Number of channels (servers)
- Queue discipline e.g. FIFO, FILO, etc.
Arrival and Departure Patterns
Arrival Patterns (λ, in veh per unit time) • Equal time interval • Exponentially distributed time intervals Departure Patterns (μ, in veh per unit time) • Equal time interval • Exponentially distributed time intervals
Queue Models
Identified by three alphanumerical values: • Arrival rate assumption • Departure rate assumption • Number of departure channels D (uniform/deterministic) M (exponential/random)
Types of Queues: • D/D/1 • M/D/1 • M/M/1 • M/M/N
D/D/1
- When the arrival curve is above the departure curve, a queue will exist.
- The point at which the arrival curve meets the departure curve is the moment when the queue dissipates.
- The point of queue dissipation can be determined by equating appropriate arrival and departure equations.
- Under the FIFO queuing discipline, the delay of an individual vehicle is given by the horizontal distance between arrival and departure curves
- The total queue length is given by the vertical distance between arrival and departure curves at that time
- The total area between arrival and departure curves gives total vehicle delay.
M/D/1
The assumption of exponentially distributed times between the arrivals of successive vehicles (Poisson arrivals) will, in some cases, give a more realistic representation of traffic flow than the assumption of uniformly distributed arrival times.
Average number in the system: E(n)= (ρ(2-ρ))/(2(1-ρ)) Average queue length: Q ̅=ρ^2/(2(1-ρ)) Average time in the system: w ̅=ρ/(2μ(1-ρ)) Average waiting time in the queue: t ̅=(2-ρ)/(2μ(1-ρ))
M/M/1
A queuing model that assumes one departure channel and exponentially distributed departure times in addition to exponentially distributed arrival times is applicable in some traffic applications.
Average number in the system:
E(n)= ρ/(1-ρ)
Average queue length (veh):
Q ̅= ρ^2/(1-ρ)
Average time in the system:
t ̅=1/(μ- λ)
Average waiting time in the queue:
w ̅= λ/(μ(μ-λ))
Probability of the queue being empty:
P_0=1-ρ
Having to wait:
1- P_0=ρ
(n) units in the system:
P_n=(1-ρ)ρ^n
More than N times in the system:
Pr(n>N)= ρ^(N+1)
M/M/N
M/M/N queuing is a reasonable assumption at toll booths and toll bridges, where there is often more than one departure channel available. Note that in this case ρ might be greater than 1, but ρ/N should be less than 1, where N is the number of channels.
Probability of having no vehicles in the system:
P_0=1/(∑_(n_c=0)^(N-1)▒〖ρ^(n_c )/(n_c !)+ ρ^N/(N!(1-ρ/N))〗)
Probability of having n vehicles in the system (for n ≤ N):
P_n=(ρ^n P_0)/n!
Probability of having n vehicles in the system (for n ≥ N):
P_n=(ρ^n P_0)/(N^(n-N) N!)
Probability of waiting in a queue (the probability that the number of vehicles in the system is greater than the number of departures channels):
P_(n>N)=(ρ^n P_0)/(N!N(1-ρ/N))
Average length of queue (in vehicles):
Q ̅= (ρ^n P_0)/N!N [1/〖(1-ρ/N)〗^2 ]
Average waiting time in the queue, in unit time per vehicle:
w ̅=(ρ+ Q ̅)/λ-1/μ
Average time spent in the system, in unit time per vehicle:
t ̅= (ρ+ Q ̅)/λ
Intersection
Facilitate the operation of traffic with safety and efficiency, taking into account the needs of different categories of road users.
Intersection Principles
- Understand human, vehicle, road factors
- Provide adequate sight distances
- Provide adequate warning of the intersection
- Ensure that the layout is easily recognized
- Accommodate appropriate vehicle speeds
- Give preference to major traffic movements
- Minimize the number of conflict points
- Provide adequate facilities for all road users
Intersection Types
- Signalized, unsignalized or roundabout
- Channelised or unchannelised
- Flared, unflared or auxiliary lanes
- Urban or rural
Types of Control
Primary options • Road rules only • Give way lines only • Stop and give way signs • Roundabout • Traffic signals
Other options
• Ban movements
Basic Design Considerations
- Design Vehicle: the largest vehicle likely
- Visibility for vehicles: at each conflict point
- Queuing through intersections: not ideal
- Sight Distance: drivers must be able to see the path they have to follow
- Merge Behaviour: merge from the left
- Reducing points of conflict: controlling or separating movements
Conflict Points
32 Potential Points
Unsignalised Intersections
Priority between conflicting traffic movements:
• Application of the road rules
• Regulatory devices, such as stop or give way signs
• Physical devices, such as traffic islands or medians
Key Traffic Management Considerations
Used when low volumes and low speeds occur both in urban and rural locations. Compact and low cost, any road surface. Treatments for safety reasons.
Assessment of Delays and Queues
- Traffic surveys
- Analytical methods based on gap acceptance criteria and absorption capacity of the major flows
- Analytical computer programs (SIDRA)
- Mirco-simulation programs (e.g. AIMSUN, VISSIM) for complex situations such as staggered T-intersections.
Analytical Methods
- Qld TMR: Road Planning and Design Manual – Chapter 13: Intersections at Grade
- Austroads: Guide to Traffic Management – Part 6: Intersections, Interchanges and Crossings
TMR- Acceptable Gap Distance - TMR Method
Practical Absorption Capacity
Determine major stream flow (qp)
Choose critical gap (ta) and follow-up headway (tf)
Determine practical absorption capacity (Cp)
C=(q_p e^(〖-q〗_t t_a ))/(1-e^(〖-q〗_p t_f ) ) C_p=0.8C Where: t_a=Critical gap t_f=Follow-up headway C=Theoretical absorption capacity
TMR - Critical and Follow up Headway
See Notes
TMR - Average Delay
- Determine practical absorption capacity (Cp)
- Determine minor flow (qm)
- Calculate the number of required lanes on the minor stream (n)
- Determine average delay (Wm)
See Notes of more detail
TMR - Storage Requirements
Determine minor flow (qm)
Determine maximum service rate to the minor stream (qs = C = Cp/0.8)
Choose a level of probability that the design queue length will not be exceeded (e.g.95%)
Calculate storage spaces required (storage length 8m for each vehicle)
Pr(>N)= ρ^(N+1)
Austroads - Absorption Capacity (Service Rate)
C=(q_p e^(〖-q〗_t t_a ))/(1-e^(〖-q〗_p t_f ) ) C_p=0.8C Where: t_a=Critical gap t_f=Follow-up headway C=Theoretical absorption capacity
Average Delay
See Notes
Intersection Design Software
Analytical models (e.g. SIDRA) • Use ‘traffic flow theory’ to determine delays, queues etc. Simulation models (e.g. VISSIM, AIMSUN Paramics) • Consider interactions of individual vehicles in small time slices
Roundabouts
8 Conflict Points
Based on circulating lanes and gap acceptance
Appropriate Sites for Roundabouts
- Where traffic volumes result in unacceptable delays for the minor road traffic
- High proportions of right-turning traffic, with entering flows reasonably balanced
- Rural cross intersections with a crash problem
- Where traffic speeds are high and right turning traffic flows are high
- Where traffic growth is expected to be high
Inappropriate Site for Roundabout
- Where geometrics cannot be provided
- Where there are very high traffic flows or they are unbalanced
- Considerable pedestrian activity
- Located on a major on road cycle route
- An isolated intersection in a network of linked signals
- Where large combination vehicles or over dimensional vehicles use the intersection
Signalized Intersections - Key Traffic Management Considerations
- Most suitable for very high volume sites
- Enables efficient traffic coordination
- Readily accommodate PT priority
- Controlled crossings for pedestrians and cyclists
- Safer for cyclists than multi-lane roundabouts
- Preferred for high pedestrian activity
- Preferred to roundabouts for freight routes
- Not as safe as a roundabout
Interchanges
Interchanges must be provided where:
• Major intersecting road is a motorway
• An economic analysis demonstrates that it is justified
Interchanges - Key Traffic Management Considerations
- All turning movements via ramps
- Generally at-grade intersections provided at ramp terminals
- No local access for vehicles, pedestrians or cyclists
- Network issues for downstream intersections in an arterial system
- Accommodations for pedestrians and cyclists
Interchange Forms
• System interchanges –provide for uninterrupted flow between major roads
o Few freeway – freeway “stacks”
• Service interchanges – provide connections between a motorway and an arterial
• Cloverleaf interchange
• Stack interchange
Roundabout
A channelized intersection at which:
• All traffic moves round a central traffic island; and
• Circulating traffic has right of way.
Roundabouts History
Traffic circle’, ‘Rotary’ or ‘Gyratory’ o First used in US early last century o Rely on weaving maneuvers o Large diameter – higher speeds • Modern roundabouts: o Started in the UK in 1950s o Exported to Australia and France in 1970s o Others followed – US in 1990 o Rely on gap acceptance – lower speeds
Roundabout Anatomy
- Leg
- Approach carriageway and width
- Kerb blister
- Give-way line
- Splitter island
- Central island and central island radius
- Circulating carriageway and width
- Departure carriageway
See Notes for Diagram
Roundabout Application
Can be used at a wide range of sites: • Urban intersections • Rural intersections • Freeway interchanges and terminals Best suited: • At the intersection of roads with similar flows • Where the proportion of turning traffic is high • At multi-leg intersections
Inappropriate Sites - Roundabout
Roundabouts may be inappropriate where:
• Satisfactory geometry cannot be provided
• Traffic flows are unbalanced
• Major/minor roads intersect
• There is considerable pedestrian activity
• A linked traffic signal system operates
• Tidal flow lanes are/may be in operation
• Large oversized vehicles are frequent
• Exiting vehicles may queue back into the roundabout.
Urban Form Benefits
A properly designed roundabout can enhance the urban form by:
• Acting as a ‘threshold” to specific precincts
o Establish a “sense of arrival”
o Provide opportunities for an ‘entry statement’
• Signal drivers of a change in driving environment
o Reduce vehicle speeds
o Alert drivers to the likely presence of pedestrians
Guidelines - Roundabout
See Notes for Table
Safety
A well-designed roundabout can be a very safe type of intersection control. This is because:
• Fewer conflict points
• Fewer and more simple driver decisions
• Lower relative speed of conflicting vehicles
Conflict Points
Crossing
Weaving
Merging
Diverging
Pedestrians - Roundabout
Positive:
• Pedestrians only required to cross one stream at a time
Negatives:
• Unlike signals, roundabouts do not give priority to pedestrians over cars
• Exits are particularly hazardous
• Pedestrians have special design requirements
Cyclists - Roundabout
Roundabouts increase the risk of accidents to cyclists, however this is not such an issue on single lane roundabouts where speeds are <50km/h. Cyclists also have special design requirements.
Roundabout Special Treatments
• High left turn flows o Left turn slip lanes • High through or right turn flows o Grade separation • High volumes/delays o Signalization • Limited space o Mountable central island o Mini-roundabout • Trams o Through the middle
Roundabout Design Principles
Provide sufficient capacity • For the largest vehicle (design vehicle) • For all vehicles (design volumes) Control speeds • Ensure adequate deflection • Separate approaches Ensure adequate sight distance • Three separate criteria
The Design Process - Roundabout
- Select design vehicle
- Determine number of lanes
- Determine size of central island
- Determine width of circulating roadway
- Check deflection
- Check sight distance
- Design super-elevation and drainage
- Design lighting
Number of Lanes - Roundabout
• Should be the minimum required to achieve desired operating performance
o Accident rates and construction costs increase with number of lanes
• Number of circulating lanes must be ≥ number of approach lanes:
o Number of approach lanes can vary between approaches
Roundabout Diameter
Larger roundabouts:
• Facilitate better design
• Allow greater separation between approaches
• Require more land
• Cost more to build
Can result in higher relative speeds of vehicles
Swept Path
• The path inscribed by the extremities of a vehicle as it performs a particular manoeuvre
• The size of the swept path increases:
o With the size of the vehicle
o Inversely with the radius of the turn
• The shape of this path can be obtained via:
o Fixed turning templates
o Specilaised CAD software
Entry Lane Width - Roundabout
• Directly effects vehicle swept paths through the roundabout
• Smaller widths result in smaller turn radii, better deflection and lower speeds
• Generally 3.4-4.0m
• Up to 5.0m for kerbed single lane entries:
o Allows traffic to pass a disabled vehicle
• Need to be checked against vehicle swept paths
Curvature - Roundabout
• Entry curve is one of the most important geometric elements of good roundabout design
• A left entry curve must be used
o Or right hand curve when driving on the right
• An appropriate radius encourages drivers to slow down before reaching the roundabout
o Should be chosen so that the 85th percentile speed is less than 60km/h
Deflection - Roundabout
• Important to maintain safety, by reducing speed of entering vehicles • Achieved by: o Alignment of approach carriageway o Splitter islands o Size and position of central island o Staggered entry/exit carriageways
Sight Distance Criteria - Roudnabout
Sight Distance – Criteria 1:
• Approaching drivers must be able to see splitter island, give-way line, central island and desirably the circulating carriageway.
• Measured from driver eye height (1.15m) to object height (0m)
• Minimum distance:
o 30m at 40km/h
o 105m at 80km/h
o Table 14.6 in Main Roads Road Planning & Design Manual (RP&DM).
Sight Distance – Criteria 2:
• Drivers at give-way line must see traffic on previous approaches
• Measured from
o 5m back from give-way line
o Driver eye height to driver eye height
o The conflict point around to the previous approach
• Minimum distance = 4 sec of travel at the 85th percentile speed (on the roundabout) + stopping distance
• For the approach immediately to the right:
o Use 85th percentile speed on previous approach (or Table 14.7 in RP&DM)
• For other approaches:
o Use 85th percentile speed of circulating traffic
o Or Table 14.8 in RP&DM
Sight Distance – Criteria 3:
Approaching drivers must be able to see traffic on any previous approach before reaching the give-way line. The objective here is to minimize vehicle delay:
• Drivers will not have to stop at the give-way line if they know in advance that there are no other vehicles on or approaching the roundabout
Measured:
• From driver eye height to driver eye height
• From altered stopping distance, based on 85th percentile speed on entry curve, plus 5m from give-way line (refer Table 14.6 RP&DM)
This criterion may not always be possible to achieve in urban areas.
Super-Elevation - Roundabout
- The tilting on a carriageway to help offset the centripetal forces developed as a vehicle traverse a curved path. It works with friction to keep vehicles from going off the road.
- Often conflicts with drainage requirements
- Super-elevation on the circulating roadway can sometimes hide the roadway and central island from the view of approaching drivers
- 0.025-0.03m/m should be adopted for circulating roadway
- Min 0.02m/m adequate for drainage provided construction tolerances are tightly controlled.
Lighting - Roundabout
- Need to illuminate raised splitter islands, central island, circulating roadway and approaches
- Main Roads Road Planning & Design Manual Chapter 17.
Pedestrians - Roundabout
• Splitter islands are used to shelter pedestrians
• Avoid painted ‘zebra’ crossings – use pram ramps instead
o 6-12m from give-way line of approach
o 12-24m on departure side
• Minimize entry/exit speeds using small radius entry/exit curves
• Never direct pedestrians across central island
Cyclists - Roundabout
Roundabouts increase the crash risk for cyclists
The key is to:
Reduce relative speed of entering and circulating vehicles
Minimize the number of circulating lanes
Maximize separation between approaches
Best to provide a separate cyclist path outside of the circulating carriageway
Signalization reduces cyclist crash rates
If risks too high, consider other forms of control
Specific provision for cyclists not required on single lane roundabouts
Special provision required when:
∑▒〖approach volumes>10,000 vpd〗
Multi-lane roundabout
Vehicle speeds > 50km/h
Special Treatments
• Particular problems occur at locations where:
o One intersecting street is considerably wider than the other
o ‘T’ intersections
o There are multiple approach roads
Wider Streets
- Non-circular central island
- Kerb blisters
- Curvilinear entry
T-Intersections - Roundabout
Solution:
• Kerb blisters
• Line-marking
• Rrpm’s
Multiple Approaches - Roundabout
Solution: • Stick to the fundamentals o Control speeds o Ensure adequate deflection o Separate approaches
Objective - Roundabout
Quantify the operational characteristics of a roundabout under design conditions: • Physical layout/geometry • Design turning volumes Key characteristics: • Degree of saturation • Vehicle queuing • Delay
Degree of Saturation
- Arrival flow/entry (absorption) capacity
* Ranges from 0.0 (good) to 1.0 (bad)
95th Pecentile Queue
• The number of vehicles in a queue that is only exceeded 5% of the (design) time
Queuing Delay
• Delay to vehicles as they wait to accept gap in the circulating traffic stream
Geometric Delay
- Delay to vehicles as they slow down to negotiate the roundabout and speed up again upon exiting
- Different for vehicles that stop and don’t stop.
Factors affecting Roundabout Performance
- Traffic volumes
- Proportion of heavy vehicles
- Number of entry lanes
- Average entry lane width
- Number of circulating lanes
- Roundabout diameter
- Platooning from upstream traffic signals
Analysis Method
• Manual
o AUSTROADS Guide to Traffic Engineering Practice – Part 6 (Chapter 3)
• Analytical computer models
• Simulation computer models
Analytical Process - Roundabout
Based on standard gap acceptance techniques:
• Treat roundabout as a series of priority controlled ‘T’ intersections
• Each lane of each approach analyzed separately
• Gap acceptance parameters determined by roundabout geometry and demand flows:
o Geometry that offers easier entry path will have lower gap acceptance values
o As circulating flow increase, circulating speeds decrease and drivers are more willing to accept smaller gaps
• Overall performance is determined by the most congested entry lane
Key Assumptions - Roundabout
• Entering vehicles give way to circulating vehicles
• At multi-lane entries, vehicles can enter simultaneously
• Drivers entering on different lanes of the same approach behave differently
o Dominant vs sub-dominant lanes
• Exiting vehicles have no effect on entering vehicles
Traffic Signals - Objective
To improve safety and efficiency through: • Reducing delays • Reducing traffic conflicts o Number of conflict points o Type o Shared time
Warrants
• Guide
• References:
o TMR Manual of Uniform Traffic Control Devices (MUTCD)
o FHWA Manual of Uniform Traffic Control Devices (MUTCD)
TMR Warrants
Traffic Demand Volumes • 4 x 1hr major > 600 veh/hr • Minor > 200 veh/hr Continuous Traffic • 4 x 1hr major > 900 veh/hr • Minor > 100 + speed + SD Pedestrian Safety • 600 veh/hr + 150 ped/hr or 400 where 85% + 75km/hr Crashes • 3+ casualties or towing p.a. for 3 years Combined Factors • 80% of 2 or more of the above warrants
FHWA Warrants
Warrant 1: Eight-Hour Vehicular Volume Warrant 2: Four-Hour Vehicular Volume Warrant 3: Peak Hour Warrant 4: Pedestrian Crossing Warrant 5: School Crossing Warrant 6: Coordinated Signal System Warrant 7: Crash Experience Warrant 8: Roadway Network Warrant 9: Intersection near a Grade Crossing
Warrant 1
Eight Hour Vehicular Volume
Warrant 2
Four Hour Vehicular Volume
Warrant 3
Peak Hour
Warrant 4
Pedestrian Volume
Warrant 5
School Crossing
Warrant 6
Coordinated Signal System
Warrant 7
Crash Experience
Warrant 8
Roadway Network
Warrant 9
Intersection near a Grade Crossing
Design Data for Traffic Signals
Three conditions need to be analyzed before a signalized intersection can be installed:
• Geometric Conditions
• Traffic Conditions
• Signalization Conditions
Traffic Signal Cycle
Cycle: Once complete sequence (for all approaches) of signal indications (GYR)
Traffic Signal Cycle Length
Cycle Length: The total time for the signal to complete one cycle (C in seconds)
All Red
All Red: The time within a cycle in which all approaches have a red indication (AR)
Green, Yellow and Red time
Colour Phasing
Phase
Phase: The sum of displayed green, yellow and all red times for a movement or combination of (compatible) movements that receive the right of way simultaneously during the cycle. The sum of phase lengths is the cycle length.
Signal Design Components
Signal Design Components: 1. Phase plan – number, sequence. 2. Type of signal controller 3. Signal phasing 4. Signal timing • Cycle timing • Green time • Yellow time • Red time
Phase Plan
See Notes
Permitted Movements
Permitted Movement: Vehicles that make their way through gaps in a conflicting vehicle movement.
Protected Right turn
Protected Right Turn:
Leading:
• Precedes the opposing straight through movement
Lagging:
• Follows the opposing straight through movement
Overlap Movement
A movement that occurs in more than one phase and continues between phases.
Control Modes
- Pretimed (fixed time): No real-time inputs from traffic flow, preset phases or sequence and duration (peak and off peak)
- Semi-actuated: Vehicle detection on some approaches
- Fully actuated: vehicle detection on all approaches
Actuated Phase Intervals
See Notes
Saturated Flow Rate
Saturation Flow Rate is the maximum hourly volume that can pass through an intersection, from a given lane or group of lanes, if that lane (or lanes) were allocated constant green over the course of an hour. Saturation flow rate can be determined by:
- Measurement
- Prediction models
Measuring Saturation Flow Rate
s=1/h
Where:
s = Saturation flow rate in veh/hr; and
h = Saturation headway in hr/veh (also called discharge headway)
Discharge Headway
The time interval separating two consecutive vehicles passing a stop line in a one lane traffic stream. Measured at rear wheels of vehicles:
• First headway includes driver reaction time, and acceleration time;
• Second headway has shorter headway;
• Eventually headways become equal after 4-5 vehicles; and
• Saturation headway is measured at stable conditions.
Saturation Flow Rate
- Saturation headway, h (hr)
- Saturation Flow Rate (100% green time)
- Multiple lanes s (vphgpl)
- Default vale for base saturation flow rate is 1950 veh/h/lane.
Capacity
Saturation Flow rate, s (veh/h): Assuming the signal always green, maximum hourly volume
Portion of saturation flow used: portion of cycle which is effectively green
Lost Time
t_L=t_sl+t_cl Where: t_L= Total lost time for a movement during a cycle (s) t_sl= Start-up lost time (s) t_cl= Clearance lost time (s)
Effective Green
g=G+Y+AR-t_L
Where:
g = Effective green time for a traffic movement (s)
G = Displayed green time for a traffic movement (s)
Y = Displayed yellow time for a traffic movement (s)
AR = All-red time for a traffic movement (s)
t_L = Total lost time for a movement during a cycle (s)
Effective Red Time
r=C-g
r=R+t_L
Protected Right Turn Important Factors
Important Factors: • Volume • Delay • Queue Length • Opposing traffic speeds • Geometry (e.g. sight distance) • Crash Experience
Critical Lane Group
There is one critical lane group among lane group movements in each phase
When there is no overlap movement, the CLG is the LG with the highest flow ratio
Flow ratio is the ratio of arrival rate to departure rate [equal to intensity (ρ) in queuing theory]
Flow Ratio=v/s
v= Arrival flow rate
s= Saturation flow rate
Traffic Signals Performance Measurements
- Degree of Saturation (DoS): ratio of demand to capacity
* Delay due to traffic signal control
Money Over Time
- Money now has a different value than the same amount at a different date
- Money can be used to make more money over time
- Future costs/revenues are reduced (discounted) as compared to present
- Cash flow analysis is performed to calculate the equivalent of money at different times
Benefit-Cost Analysis
- Benefit-cost analysis is a systematic approach to estimating the strengths and weaknesses of alternatives
- Benefit-cost analysis is a purely quantitative approach to decision making
- Benefit-cost analysis might incorporate monetized forms of non-financial performance measures
- Benefit-cost analysis does not incorporate qualitative measures
- Benefit cost analysis is not subjective
Benefit-Cost Ratio Test
- Calculate B/C ratio for all the projects, then choose the project with the largest B/C ratio.
- B/C can be calculated for any form of benefits and costs as long as they can be quantified (monetized)
- B/C can be applied to benefits and costs in either present, future of uniform series cash flows
Net Present Value Test
- Calculate NPV of all the projects and choose the one with the largest B/C ratio
- NPV test can be calculated for any form of benefits and costs as long as they can be quantified (monetized)
- NPV test is applied to benefits and costs only is present value
- NPV is a good complementary measure when combined with B/C test, in the sense that it gives some idea about the scale of the net benefit, as opposed to benefit ratio cost.
Cost Effectiveness Test
• A cost-effectiveness analysis expresses the result in terms of the average cost per unit of effectiveness (e.g. the average cost per life saved)
• Calculate cost per unit of effectiveness for each project, then choose the project with the smallest cost per unit.
• Cost-effectiveness test can be calculated for any form of costs as long as they can be quantified e.g. time, resources etc.
• Cost-effectiveness is usually used when:
o The effectiveness of the project cannot be easily quantified
o The projects have a single goal to achieve
Classifications of Cost-Effectiveness Test
Alternative forms of cost-effectiveness analysis:
• Total cost/units of effectiveness
o E.g. the average cost per saved life
o Choose the project with the smallest cost per unit of effectiveness
o Advantage: it is a measure specified in dollars
• Total effectiveness/cost of project
o E.g. saved lives by a dollar
o Choose the project with the greatest effectiveness per dollar
o Advantage: it is consistent with the term “cost-effective”
Multi-Criteria Analysis
• The benefit-cost analysis is a useful method to evaluate projects when all the performance measures can be monetized
• When qualities are difficult or impossible to quantify/monetize then use MCA. Some difficult qualities to quantify include:
o Social:
• Equity, public health, safety, crime, human rights, diversity, history and heritage, arts and culture, quality of life, education, etc.
o Environmental:
• Emissions, noise, aesthetics, etc.
o Personal:
• Ethics, comfort, convenience, happiness, etc.
• The MCA methods are flexible along many directions:
o Choice of alternatives
o Choice of performance criteria
o Weighting of performance criteria
o Evaluation of the performance measures (quantification of scoring)
o Who is involved in weighting and scoring
• Advantages:
o Can incorporate a diverse range of information and measures
• Prospects, preferences, and perceptions of the beneficiaries
• Monetary, monetized, quantified and quality measures
o Can be communicated easily
o Flexible along many dimensions
• Disadvantages:
o Weighting is hard to derive
o Lacks methodological rigor
Steps for MCA
- Identify Alternatives
- Establish Performance Criteria Set
- Establish relative importance of performance criteria (weighting)
- Establish scales for measuring levels of each criterion (for unquantifiable measures)
- Quantify the level of each criteria based on the scales (scoring)
- Normalize the scores if necessary
- Calculate the combined impact of different criteria for all alternatives
- Determine the most satisficing alternative
Methods use for weighting and scoring
- Direct weighting and scoring
- Questionnaire/interview
- Pairwise comparison
- Delphi technique
- Regression-based (based on observation)
Domination of Alternatives
Alternative A is dominated by alternative B if:
• Alternative A is worse than or equal to alternative B in all criteria, and
• Is strictly worse than B in at least one criterion
Managing Traffic Congestion
- Traffic congestion is a condition on road networks that is characterized by slower speeds, longer trip times, and increased vehicular queuing
- Congestion happens when traffic demand exceeds roadway supply
Adverse Effects of Congestion
- Delay in traffic
- High energy consumption
- Effects on environment
- Safety aspects
- Emergency access breakdowns
- Poor quality of life
Managing Traffic Congestion
Supply
• Roadway network, facilities, equipment
• Access to the network (traffic operation)
Demand Decisions:
• Whether to drive
• Where/when to go
• Which path to take
Managing Supply: • Taking advantage of the existing resources in the most efficient way • Examples: o Intersection Control • Traffic signal timing • Traffic signal coordination • Emergency phases o Roadway lane control • Reversible lanes • HOV (high occupancy vehicle) lanes • Truck lanes o Traffic Access Control • Ramp metering, turn prohibition, etc. o Incident detection and clearance
Managing Demand:
• Influencing driving choices to improve the situation
Intersection Control - Traffic Signals
- Traffic signal timing: optimize the times allocated to signal phases of an intersection, to facilitate the most efficient flow of traffic
- Pre –timed vs. actuated
Actuated signals require traffic detecting devices
• Loop detectors (loop induction devices)
• Traffic cameras
• Pressure pipe devices
Traffic Signal Coordination
- A series of consecutive traffic lights on a street can be coordinated to allow continuous flow of traffic in one main direction
- A platoon of vehicles can travel in a “green wave” without having to stop at any red lights
- Reduction in intersection delay, fuel consumption, emissions, noise and congestions.
- Controls the speed of the traffic as well
- Traffic signal coordination in one direction is easy when the cycle lengths are equal:
Emergency Vehicles Phases
- Opticom infrared detectors: Emergency vehicles (ambulances, police cars, fire trucks etc) approaching the intersection trigger the device
- An all red phase facilitates fast procession of the emergency vehicle
Managing Supply
Roadway lane control:
• Reversible lanes, barrier transfer machines (zippers)
• Truck prohibited lanes/roads
• Variable speed lanes improves safety
• Traffic access control
• Ramp metering
o The idea is to maintain the LOS of the motorway at a certain level
o Maintain the maximum flow on the motorway, by limiting the on-ramp flow to the motorway
• Incident detection and clearance
o An ‘incident’ is defined as: any non-recurring event that causes a reduction of roadway capacity
o With quick clearance and recovery, drastic degradation of traffic flow can be avoided
Managing Demand
Demand management: influence the driving choices to improve the traffic situation. Driving choices include: • Whether to drive? o Alternative modes/mode shares • Where/when to go? o Alternative activity locations o Departure time • Which path to take? o Distribute traffic o Avoid high peaks
Encourage non-motorized modes:
• High speed public transit
• Transit discounted fares
• Transit comfort and convenience
• Transit accessibility and reliability improvement
• Bike accessibility and safety improvement
• Walkable streets, accessible walks, sidewalks and pedestrian crossings
Discourage driving: • Tolled motorways, bridges • Cordon and congestion pricing • Parking policies • Gas prices (expensive fuel) • Eliminate traffic peaks o Flexible school/work hours o Tolls, congestion pricing, dynamic tolls o Carpool policies and HOV’s o Traffic advisory information
