Module 9: The Science of Traffic Flashcards
Elements of Traffic Engineering.
- Traffic studies & characteristics
- Performance evaluation
- Traffic control
- Traffic operations
- Transportation systems management
- Intelligent transportation system
Traffic Stream.
-The study of the movements of individual drivers & vehicles between two points & the interactions they make with one another.
-The traffic flow characteristic shows the dynamic change at different time intervals through the day.
-The traffic flow theory classified the traffic movement in the network into two main streams, interrupted & uninterrupted traffic flow.
Uninterrupted & Interrupted Flow.
-Uninterrupted flow facilities have no external interruptions to the traffic stream.
-Interrupted flow facilities are those that incorporate fixed external interruptions into their design & operation
-Major difference between the two flow facilities is the impact of time
Traffic Flow - Basic.
-The more vehicles are on a road, the slower their speed will be.
-When the number of vehicles entering a zone > the number of vehicles leaving the zone, congestion occurs.
-Traffic flow will change from stable to unstable when at a critical density & a corresponding velocity
-If one vehicle breaks in unstable flow, the flow will breakdown
Traffic Flow Parameters.
-Many variables play a fundamental role in characterizing, modelling, & studying the dynamic of traffic.
-The most important variables are:
Flow = Density x Speed
Density.
-The number of vehicles occupying a length of a lane or roadway at a particular instant = veh/km
Speed.
-The rate of change of its position = km/h
Flow.
-The number of vehicles passing a point on the road per specific time = veh/h
Speed vs Density.
-Vehicle density affects speed
-As the number of cars increases, the speed decreases
Traffic Flow Parameters - Flow.
-Flow: The amount of traffic passing a point or on a lane or roadway during a specified time interval.
-It can be expressed as veh/day or veh/hr
-Maximum flow is qmax
-It represents the demand
-It helps classify roadways; planning & design roadways
The U-Bend of Flow.
-Basis for uninterrupted flow
-A ‘0’ flow rate occurs under 2 different conditions:
-when density is zero
-when density is so high that all vehicles stop
Speed vs Flow.
-Vehicle density affects speed (non-linearly)
-Flow = Density x Speed
-Managing vehicle density keeps speeds & flow high
Why is there Traffic Jams without Obvious Reason?
-Breakdown when Density > Dcap
-But ‘stop and go’ occurs sometimes when LOS is better than F & there is no crash or blocked lane
Traffic Shockwaves.
-Shockwaves are created under the conditions of traffic congestion & queuing.
-A shockwave propagates along a line of vehicles in response to changing conditions at the front of the line.
-Shockwaves that occur in traffic flow are similar to the waves produced by dropping stones in water
-Shockwaves can be generated by collisions, sudden changes in speed, or by a number of other means.
-Increasing vehicle headway & reducing the use of brakes can help solve it.
Traffic Flow Parameters - Speed.
-Speed = Distance / Time
-Rate of change of its position
-Free flow speed is designated by Vmax
-In a moving traffic stream, each vehicle travels at a different speed. Thus the traffic stream has a distribution of individual speeds
-The traffic stream, taken as a whole, can be characterized using an average or typical speed
Traffic Flow Parameters - Density.
-Density = Flow / speed or # of vehicles / length
-The number of vehicles occupying a given length of a lane or roadway at a particular instant.
-It is important because it is the measure most directly related to traffic demand
-It is also an important measure of the quality of traffic flow
-Drivers select speeds that are consistent with how close they are to other vehicles. The speed & density combine to give the observed rate of flow.
Traffic Demand.
-The number of vehicles that desire to pass a point or segment of a roadway in an hour. (veh/h)
Traffic Volume.
-The number of vehicles that actually pass a point or segment of a roadway in an hour. (veh/h)
Capacity.
-The maximum volume (or rate of flow) that a particular point or segment of a facility can accommodate. (veh/h)
Demand vs Capacity.
-Traffic demand < capacity, volume = demand
-Traffic demand > capacity, volume < demand
Capacities.
-1 lane of freeway carries 2200 pc/h
-1 lane of an arterial street carries 800-900 pc/h
-Auto occupancy of 1.1-1.5 passenger/veh
-1 bus lane handles 100 buses/h or 5000 passengers/h
-LRT capacity = 20000 passengers/h
Annual Average Daily Traffic.
-The average 24 hour volume at a given location over a full 365 day year; the number of vehicles passing a site in a year divided by 365 days
Annual Average Weekday Traffoc.
-The average 24 hour volume occurring on weekdays over a full 365 day year; the number of vehicles passing a site on weekdays in a year divided by the number of weekdays (260)
Average Daily Traffic.
-The average 24 hour volume at a given location over a defined time period less than one year; a common application is to measure an ADT for each month of the year
Average Weekday Traffic.
-The average 24 hour weekday volume at a given location over a defined time period less than on year; a common application is to measure an AWT for each month of the year
Design Traffic Volumes.
-Highway design & control must be designed to adequately serve the peak hour traffic volume in the peak direction of flow
-Peak hour volumes are sometimes estimated from projections of the AADT
-DHV (Design Hourly Volume)
-used for design purposes
-DHV = AADT X K
-typically between 10th & 50th highest peak hour volume;
commonly 30th
-DDHV (Directional Design Hourly Volume)
-often there is much more traffic in one direction than the other
-DDHV = AADT x K x D
K Factor.
-K factor is defined as the proportion of annual average daily traffic occurring in an hour
-K generally decreases with increasing development density
-K is generally highest near recreational facilities, next highest in rural & suburban areas, & lowest in urban areas
D Factor.
-Indicates the directional split
-Higher D if commuter routes
-Lower D as development density increases
Peak Hour Factor.
-PHF describes how peak the traffic volume is within the peak hour
-PHF = Peak Hour Volume / (4 x Highest 15-min volume)
-Ranges from 1 to 0.25
-PHFS in urban areas = 0.8-0.98
-PHFs > 0.95 are often indicative of high traffic volumes
-PHFs < 0.8 - highly peaked demand
Key Parameters.
-AADT - over 12 month period
-ADT - other time period
-DHV - both directions
-DDHV - one direction
-DSFR (Design Service Flow Rate) = amount of traffic in the peak 15-min in one direction = DDHV/PHF
-K - % of daily traffic during the peak hour
-D - Proportion of traffic travelling in the predominant direction
-PHF - ratio of DHV to the peak 15- min flow rate
Queuing Theory.
-Traffic queues during congested periods is a source of considerable delay & results in a loss of highway performance
-Given this, it is essential in traffic analysis to develop a clear understanding of the characteristics of queue formation & dissipation
-Queueing is not unique to traffic analysis: operation of industrial plants, retail stores (banks, fast food), service-oriented industries, call centres, etc.
Queueing Theory.
-Queue = Demand > Capacity
Vehicles Per Hour.
-A simple unit expressing the number of vehicles passing a point in an hour
-Despite its simplicity, it is used to quantify 3 very different concepts:
-demand
-capacity
-volume
-Need to clearly understand the 3 concept involved, & the critical differences between them
Level of Service.
-Level of Service is a descriptive scale that describes the quality of traffic service
-LOS is used to provide a simple language that could be used to explain complex situations to decision makers & the public
Level of Service.
-LOS A - Free flow
-LOS B - Reasonably free flow
-LOS C - Stable flow
-LOS D - Approaching unstable flow
-LOS E - Unstable flow
-LOS F - Breakdown
