Traffic Analysis Toolbox Volume VI:
Definition, Interpretation, and Calculation of
Traffic Analysis Tools Measures of Effectiveness
This chapter identifies how field measurements are processed to estimate the eight MOEs selected for further investigation in the previous chapter.
The Highway Capacity Manual to date has used single field measurable performance measures for level of service. For freeways it is density in terms of equivalent passenger cars. For two-lane rural highways it is percent time delay. For arterial streets it is mean speed of through traffic. For an intersection it is mean delay. The measurement of these performance measures in the field is described below in the following sections.
Volume/capacity (v/c) ratio cannot be measured in the field unless there is a time period when the facility is observably at capacity. [There is a queue of vehicles waiting to be served by the facility that persists for at least 15 continuous minutes.] Then the v/c for any other period can be estimated by taking the ratio of the counted demand to the observed capacity.
The FHWA publication number FHWA-PL-98-035, Travel Time Data Collection Handbook, dated March 1998, prepared by the Texas Transportation Institute provides a complete overview of techniques for gathering travel-time data (see http://www.fhwa.dot.gov/ohim/start.pdf).
There are numerous speed and travel-time measuring techniques, but they can all be grouped into three large categories according to their method of sampling the travel-time universe.
- Spot speed measurement techniques measure vehicle speeds only for a given point of geography or a given point of time.
- Vehicle tracing techniques measure vehicle travel times only for a select portion of all trips.
- Trip maker tracking techniques are similar to vehicle tracing techniques but measure traveler trip times rather than vehicle trip times.
The mechanics of employing many of these techniques are described in the Institute of Transportation Engineers publication, Manual of Transportation Engineering Studies, Edited by H. Douglas Robertson, 2000. The Transportation Research Board Publication, Highway Capacity Manual, 2000 also provides some limited guidance on travel time, speed, and delay data collection in the appendices to Chapters 15 and 16 of the manual.
Spot Speed Measurement Techniques
Spot speed measurement techniques use roadside sensors to measure the instantaneous speeds of vehicles either at specific spots of the roadway or at specific times of the day. These techniques are very cost-effective at gathering large amounts of speed data for specific segments of the transportation system but cannot provide door-to-door travel times.
Roadside sensors include in-the-road loop detectors, roadside radar, microwave sensors, video sensors, and infrared sensors. They are "location-based sampling" methods which suffer from the biases inherent in measuring speeds at a point and assuming the speed is applicable to other points on the roadway. Technological variations include: single loop detectors, double loop detectors, portable machine double hose counters, radar, lidar, microwave, and infrared sensors, and video camera sensors. Recent technological advances; such as vehicle signature and platoon signature matching, may allow measurements of elapsed time between stations.
Single loop detector occupancies are converted to speeds based upon an estimated average vehicle length during the survey period. This average length varies according to the mix of autos and heavy vehicles present on the facility by time of day and direction.
Radar guns register only the fastest vehicle speed in the platoon, so there may be some upward bias. Also, the "angle of incidence" (the angle between the road centerline and the hypothetical straight line between the radar gun and the vehicle) of roadside radar and lidar measurements affect the precision of the speed results, but usually the effect is considered negligible.
The minimum sample size is determined based upon the desired confidence interval for the mean speed. Larger variances in measured speeds require larger sample sizes to achieve a desired confidence interval for the mean speed.
Vehicle Tracing Techniques
Vehicle tracing techniques involve tracking either test vehicles or randomly selected vehicles through to determine the travel times between preselected checkpoints. Vehicle tracing techniques are an example of "trip-based" method of sampling travel times. They are good techniques for measuring trip segment travel times (a geographic portion of the traveler's total trip). However, they generally are not easily adaptable to measurement of door-to-door travel times, because of the expense and difficulty of obtaining a reasonable sample of door-to-door locations.
Vehicle tracing techniques consist of: Test Vehicle, Non-Instrumented Vehicle Tracking, and Passive Probes.
- Test Vehicle (Floating Car) Technique – The test vehicle technique is the most common travel-time collection technique employed to date. This technique consists of hiring a driver and vehicle to drive a vehicle along a preselected route and measuring the elapsed time and distance traversed. The driver is instructed to "pass as many vehicles as pass him or her" so that the vehicle is in effect driving at the median speed of traffic. Labor saving variations equip the test vehicles with distance measuring instrument (DMI) or global positioning satellite (GPS) to automate measurement and recording and to eliminate the need for a second person in the vehicle.
- Non-Instrumented Vehicle Tracking Technique – This technique uses any one of several technologies for identifying randomly selected vehicles at various checkpoints within the study area and measuring the time between appearances at each checkpoint. Vehicle tracking is different than using test vehicles, because the drivers have not been hired to do the study. The drivers may take different paths and they may make stops in between checkpoints, throwing off the travel-time computations. Technological variations include: License Plate matching, License Plate matching with matching software, and Loop detectors with vehicle signature matching.
- Passive Probe Technique – This technique requires some sort of special tracking instrumentation on the vehicles as well as the roadside. The vehicle driver is not hired to drive a particular route and goes about his or her normal business. Either readers are mounted on the road to record the time and identity of all transponder-equipped vehicles passing by, or readers are mounted in the vehicle to record the times and movements of the vehicle past each transponder location. The drivers may take different paths and they may make stops in between checkpoints, throwing off the travel-time computations. Technological variations include: Automatic vehicle location (AVL), Automatic vehicle identification (AVI), Emergency vehicle tracking, Cellular phone geolocation, and Global positioning satellite (GPS).
- Transit Vehicle Tracking Techniques – The previous vehicle tracing techniques are applicable to all vehicles, including transit vehicles. The discussion under this category focuses on the special issues involved in working with public transit agencies to monitor public transit vehicles. Most public transit operators already publish route schedules and monitor on-time performance. A few operators are able to use automated techniques for tracking vehicle movements, but most currently rely upon manual checkpoint and ride check techniques. The data is stored in varying formats in varying software formats, making electronic transmittal of data difficult.
- Truck Tracking Techniques – Trucks also can be tracked using all of the previously described vehicle tracking techniques. Tracking trucks though requires the active cooperation of the vehicle fleet owner who must consent to the placement of any special devices in the vehicle, or must transcribe manual logs and share the information with interested public agencies. Public agencies wishing to track commercial vehicles must demonstrate to the vehicle fleet owner that the owner will receive some direct benefit in return for the expense of transcribing and sharing the vehicle tracking information. In most cases, travel-time information is mixed in with sensitive proprietary information on customers, and must be manually sorted out by the operator before it can be transmitted to a public agency.
Tripmaker Tracing Techniques
Trip maker tracing techniques survey travelers either after they have completed their trip or recruit volunteers in advance to record and report their travel times as part of their daily activities.
- Retrospective Surveys – Retrospective surveys quiz the traveler about their trip travel times and experiences after the fact. The traveler is not prepped in advance, so questions must be limited to what can be reasonably remembered from the previous day's or that morning's commute. Variations explored here include: household telephone surveys, surveys of employees at their work sites, and web site/e-mail surveys.
- Prospective Surveys – Prospective surveys involve at least two contacts with each individual: one contact to recruit the individual, and a second to collect the information. A third contact may be required to deliver a trip diary form or a GPS unit to the individual to aid in recording information. Travelers can be asked in advance to note a great deal of detail about their trips, including travel times for specific segments of the trip. Technological variations include: global positioning satellite receivers/recorders, e-mail reporting, and cell phone call-in.
HCM Method for Measuring Arterial Speed
Appendix B of Chapter 15 of the Highway Capacity Manual describes the following method for measuring the mean speed of through traffic on an arterial street.
"a. Use the appropriate equipment to obtain [cumulative travel time and stopped delay time]. The equipment may be computerized or simply a pair of stopwatches.
b. Travel times between the centers of signalized intersections should be recorded, along with the location, cause, and duration of each stop.
c. Test-car runs should begin at different time points in the signal cycle to avoid all trips starting first in the platoon.
d. Some mid-block speedometer readings also should be recorded to check on unimpeded travel speeds and how they relate to FFS.
e. Data should be summarized for each segment and each time period, the average travel time, the average stopped time for the signal, and other stops and events (four-way stops, parking disruptions, etc.).
f. The number of test-car runs will depend on the variance in the data. Six to 12 runs may be adequate for each traffic-volume condition."
The Highway Capacity Manual defines delay as "The additional travel time experienced by a driver, passenger, or pedestrian." Delay is thus the difference between an "ideal" travel time and the actual travel time. Since the definition of delay depends on a hypothetical "ideal travel time," delay is not always directly measurable in the field.
If the ideal travel time is defined as the off-peak travel time, then the measured delay is the difference between the actual measured travel time during the peak period, and the actual measured travel time during the off-peak period.
If the ideal travel time is defined as travel at the posted speed limit, then the delay cannot be directly measured in the field. It is estimated by subtracting the hypothetical travel time at the posted speed limit from the measured mean travel time in the field.
HCM Method for Measuring Free-Flow Speed
Appendix B of Chapter 15 of the Year 2000 Highway Capacity Manual explains how to measure the free-flow speed for an arterial street.
"This can be determined by making runs with a test car equipped with a calibrated speedometer during periods of low volume. An observer should read the speedometer at mid-block locations when the vehicle is not impeded by other vehicles and record speed readings for each segment. These observations can be supplemented by spot speed studies at typical mid-block locations during low-volume conditions."
HCM Method for Measuring Intersection Control Delay
The Year 2000 Highway Capacity Manual (see Appendix A, page 16-90 of the HCM) describes the following field procedure for measuring and computing intersection control delay for a signalized intersection.
- The survey should begin at the start of the red phase of the lane group.
- At regular intervals of between 10 and 20 seconds (but not an interval length that is evenly divisible into the cycle length) count the number of queued vehicles.
- A vehicle is considered as queued when it approaches within one car length of a stopped vehicle and is itself about to stop.
- All vehicles that join a queue are then included in the vehicle-in-queue counts until the rear axle of the vehicle crosses the stop line.
- Simultaneously count the total number of arriving vehicles (whether they stop or not).
- At the end of the survey period, continue counting vehicles in queue for all vehicles that arrived during the survey period until all of them have exited the intersection. This step requires mentally noting the last stopping vehicle that arrived during the survey period in each lane of the lane group and continuing the vehicle-in-queue counts until the last stopping vehicle or vehicles, plus all vehicles in front of the last stopping vehicles, exit the intersection. Stopping vehicles that arrive after the end of the survey period are not included in the final vehicle-in-queue counts.
The time in-queue per vehicle is equal to 90 percent of the interval between queue counts multiplied by the sum of the vehicles in queue each interval divided by the total arriving vehicles. The 90 percent factor is intended to correct for the tendency of this method to over count delay.
- D = Average control delay per vehicle (secs).
- TQ = Time in-queue per vehicle (secs).
- I = The interval (length of time) between queue counts (secs).
- Q(i) = The number of vehicles in queue at time point "i."
- V = Total arriving volume of vehicles (whether or not queuing).
- CF = Correction Factor to convert stopped delay to control delay.
- VS = Number of arriving vehicles stopping.
The correction factor is determined from the following look-up table taken from Exhibit A16-2 of the HCM (see Table 12).
<= 7 Vehicles
> 45 mph
Source: Exhibit A16-2, Highway Capacity Manual, Transportation Research Board, 2000.
According to the ITE Manual of Transportation Engineering Studies, the macroscopic approach to measuring queues is to count the arrival and departure volumes for facility aggregated to five-minute intervals. The count should start before any queues are present and it should not end until the queues have all cleared. The difference between the cumulative five-minute arrivals and the five-minute departures is the number of vehicles in queue. The arrival data must be counted just upstream of the end of the longest expected queue.
For a microscopic analysis, the license plate, arrival time, and departure time of each vehicle is recorded. The number of vehicles in queue at any point in time is the difference between the cumulative number of arrivals up to that point minus the cumulative departures up to that point in time.
The Year 2000 Highway Capacity Manual (HCM) defines a Queue as: "A line of vehicles, bicycles, or persons waiting to be served by the system in which the flow rate from the front of the queue determines the average speed within the queue. Slowly moving vehicles or people joining the rear of the queue are usually considered part of the queue. The internal queue dynamics can involve starts and stops. A faster-moving line of vehicles is often referred to as a moving queue or a platoon."
The HCM defines the Back of Queue as: "The distance between the stop line of a signalized intersection and the farthest reach of an upstream queue, expressed as a number of vehicles. The vehicles previously stopped at the front of the queue are counted even if they begin moving."
The HCM method described above for measuring control delay at a signal also can be used to measure queues.
The number of stops is can be obtained from a floating car survey, where the number of stops during each run is recorded. The number of stops obtained in this manner is representative only of vehicles driving the same path as the floating cars (usually just through traffic on the arterial).
Similarly, the HCM control delay measurement method described above can be used to identify the number of vehicles stopping on the approach to a traffic signal.
The Year 2000 Highway Capacity Manual does not describe a method for directly measuring density in the field.
Aerial photography has been used to measure densities on freeways in several cities for the purposes of congestion monitoring (see Santa Clara VTA Annual Congestion Monitoring Reports for an example). A single photo is shot every half hour during the peak period and the number of vehicles counted between interchanges to obtain an average density representative of that half-hour period for each segment of the freeway facility. Vehicle density is NOT converted to passenger car equivalents.
Alternatively, density can be computed from loop detector measurements of speed and flow using the fundamental relation d=v/s (shown in HCM, equation 23-4, page 23-12), where "v" is the flow rate in vehicles per hour, "d" is the density in vehicles per mile, and "s" is the speed in mph.
The volume is counted manually, with temporary machine counters, or with permanent loop detectors for the desired analysis period. If an axle counter is used then an adjustment may be made to the count based on a separate truck axle count (See ITE Manual of Transportation Engineering Studies).
The spot speed is measured using any one of a variety of devices (see Measuring Speed, above). A sample size is selected to reduce the confidence interval for the true mean speed to the desired size.
The variance of travel time is not usually measured because of the expense involved. An agency may take the inverse of speed (travel time per mile) measured at loop detectors and compute the variance of the travel time per mile at the detectors.