CE 434 Lecture 5

TRAVEL TIME AND DELAY STUDIES

• Travel time and delay are two of the principal measures ofhighway system performance used by traffic engineers,planners, and analysts.

• Vehicle speed is directly related to travel time and delay and isalso used to evaluate traffic and highway systems.

• Travel-time and delay studies are conducted when the sourcesand amounts of delay occurring within the section are alsonoted.

Engineers and planners use data from travel-time anddelay studies in a number of tasks, including:

• Determining the efficiency of a route• Providing input to capacity analysis of roadway

segments• Identifying problem locations as indicated by delay• Evaluating the effectiveness of traffic operation

improvements• Providing input to transportation planning models, trip

assignment models, and route-diversion models• Providing input to economic analyses of alternatives• Generating travel-time contour maps

METHODS OF STUDY

Travel-time and delay studies may be conducted using the

(1) Average vehicle,

(2) Moving vehicle,

(3) License plate,

(4) Direct observation,

(5) Interview method.

• The first two methods require test vehicles, while the othermethods do not.

• The choice of method depends on the purpose of the study;the type of roadway segment under study; the length of thesegment; the time of day of interest; and the personnel,equipment, and resources available.

• AVERAGE VEHICLE METHOD

• The average vehicle method measures travel time;running time; distance travelled; and the type,location, duration, and cause of traffic delays along thestudy route.

• The data are recorded as the test vehicle traverses thestudy route. From these data, travel speed, space-mean speed, and running speed may be calculated.

• This method is applicable to any type of route, but ismost widely used on arterial streets with at-gradeintersections.

Time of Study

• Agencies usually study travel time and delay during thepeak hours in the directions of heaviest traffic flow. It mayalso be desirable to compare travel times, speeds, anddelays between peak and off-peak periods or between setsof other conditions.

• Some of these other conditions include good versusadverse weather and commuter versus special event traffic.

Personnel and Equipment

• The average vehicle method requires a test car and themeans to record time and distance. These can be recordedmanually or automatically.

Manual Data Collection

• Manual Data Collection requires a driver and observer/recorder,two stopwatches, and data collection forms.

• The distances between control points and the length of the totalroute may be obtained from accurate, drawn-to-scale plans ormaps or from the vehicle odometer.

Automatic Data Collection

• Several varieties of hand-held and laptop computers withaccompanying software are available to perform travel-time anddelay studies.

Field Procedure

• Before test runs begin, observers select the start point, endpoint, and control point locations along the route where theywill record time measures.

• On arterial and other types of surface streets, these locationsare usually at major intersections or other easily identifiablecontrol points.

• The choice of the near curb, or center of the intersection asthe control point should be consistent throughout the studyroute.

Test Car Technique

• The driver of the test vehicle proceeds along the study route inaccordance with one of the following techniques:

• Average-car technique: test vehicle travels according to the driver’sjudgement of the average speed of the traffic stream.

• Floating-car technique: driver “floats” with the traffic by attemptingto safely pass as many vehicles as pass the test vehicle.

• Maximum-car technique: test vehicle is driven at the posted speedlimit unless impeded by actual traffic conditions or safetyconsiderations.

• The selection of test car technique is based on the purpose of thestudy and the study team’s judgement of the technique that bestreflects the traffic stream being investigated. Most study teamsprefer the average-car technique.

Moving Vehicle Method

• Moving vehicle method provides estimates of hourly volume,average travel-time, and space mean speed.

• A number of test runs are made along the study stretch and agroup of observers record various details like record timebetween various control points, individual delays, number ofvehicles over taking the test vehicle and over taken by the testvehicle, opposing traffic.

• This method is applicable only on two way routes whereopposing traffic is visible at all times.

LICENSE PLATE METHOD

• This method produces travel time only, from which average travelspeed may be calculated once the distance between observationpoints is measured.

• A test vehicle is not required. Observers position themselves at theentrance and exit to the test section and at other major intersectionsalong the route.

• As vehicles pass the observers at each location, the observers recordthe last three or four digits of the license tag along with the time froma stopwatch. A tape recorder is useful to avoid missing vehicles.

• The tag numbers and times are then matched in the office, eithermanually or by computer, to obtain travel times. The matching islaborious if done manually.

• Computer software for matching license plate numbers is available(Center for Microcomputers in Transportation, 1992).

DIRECT OBSERVATION METHOD

• Observers at an elevated vantage point can measure travel time directly between two points a knowndistance apart.

• The method requires good visibility and is not suitable for sections greater than ½ mile in length.

INTERVIEW METHOD

• Selected individuals who are willing to cooperate may provide asatisfactory sample from which to obtain travel times and delayswithout the use of a test vehicle or observers.

• These persons are asked to record their start and end times fordesignated routes. They also record the times and durations ofdelay.

• This is a variation on the average car method, except that in placeof a single test vehicle, there are multiple test vehicles. Employeeswho drive on the job, truck drivers, and taxi drivers often makegood subjects.

• This method is useful when a large amount of data is needed inshort time (Pignataro, 1973).

• The interview method requires some training and equipment (e.g.,stopwatches) for the subjects.

• The reliability of the results may not equal that of methods thatemploy better-trained data collectors.

Intersection Delay Study

Control delay is best defined as time-in-queue delay plus timelosses due to deceleration from and acceleration to ambientspeed. The 2000 Highway Capacity Manual defines a fieldmeasurement technique for control delay using the field sheet.

Actual measurements start at the beginning of the red phase ofthe subject lane. There should be no overflow queue from theprevious green phase when measurement start. The followingtasks are preformed by the two observers;

Observer 1

• Keep track of the end of standing queues for each cycle by observing the last vehicle in each lane that stops due to the signal. This count includes vehicles that arrive on green but stop or approach within one car length of queued vehicles that have not yet started to move.

• At intervals between 10s and 20 s, the number of vehicles in queue are recorded on the field sheet.

• An the end of the survey period, vehicle-in-queue counts continue until all vehicles that entered the queue during the survey period have exited the intersection.

Observer 2During the entire study period, separate counts are maintained of vehicles

arriving during the survey period and of vehicles that stop one or more times during the survey period. Stopping vehicles are counted only once, regardless of how many times they stop.

It is than assumed that the average time-in-queue for a counted vehicle is the time interval between counts.

Then:

9.0*)*(T

iqQ V

VIsT å=

The adjustment factor (0.9) adjusts for errors that generally occur when this type of sampling techniques is used. Such errors usually result in an overestimate of delay.

A further adjustment for acceleration/deceleration delay requires that two values be computed: (1) the average number of vehicles stopping per lane, per cycle, and (2) the proportions of vehicles arriving that actually stop.

LC

STOPSLC NN

VV*

=

=SLCV=STOPV

=CN

=LN

number of vehicles stopping per lane, per cycle (veh/in/cycle)

total count of stopping vehicles

included in the survey

number of lanes in the survey lane group

number of cycles

T

STOP

VVFVS =

Where FVS = fraction of vehicles stopping (other variable as previously defined)Using the number of stopping vehicles per lane, per cycle , and the measured free-flow speed for the approach in question, correction factor is found in Table

Free-Flow Speed

(mi/h)

Vehicles Stopping Per Lane, Per Cycle

≤7vehs 8-19vehs 20-30vehs

≤37≤37-45>45

+5+7+9

+2+4+7

-1+2+5

)( SLCV

Adjustment factor for acceleration /deceleration Delay

)*( CFFVSTd Q +=

The final estimate of control delay is then computed as:

Where: d = total control delay, Sec/VehCF = correction factor from

• Table Sample Data For a Signalized Intersection Delay Study

Clock Time Cycle Number

Number of vehicle in Queue

+0secs +20secs +40secs

5:00PM5:01PM5:02PM5:03PM5:04PM5:05PM5:06PM5:07PM5:08PM5:09PM

12345678910

4632556324

7656348443

5554354335

Total 40 50 42

vehsViq 132=å vehsVT 120= 75=STOPV hmiFFS /35=

The approach has two lanes, and the signal cycle length is 60 seconds. Tencycle were surveyed, and the vehicle-in-queue count interval is 20 seconds.Sum of all vehicles in queue is 132. The average time in queue is computed :

vehsTQ /8.199.0*120132*20 =÷

øö

çèæ=

vehsVSLC 75.32*10

75==

625.012075

==FVS

Using this and the measured free-flow speed of 35 mi/h, the correctionfactor is +5 seconds. The control delay is

vehsd /9.22)5*625.0(8.19 =+=