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.
