Field Evaluation of Detection-Control System

CHAPTER 1. INTRODUCTION AND BACKGROUND

INTRODUCTION

The objectives of this evaluation study are as follows:

• Verify the detection-control system (D-CS) design objectives through rigorous field instrumentation—at the moment of signal change from green to yellow, no truck should be in the dilemma zone, and no more than one passenger car should be in the dilemma zone.

• Quantify the effectiveness of D-CS in improving safety and reducing dilemma-zone-related crashes, and red-light violations at rural, high-speed, signalized intersections.

• Identify the upper limit of traffic conditions under which D-CS can operate safely and effectively while alternative signal timing strategies may start to fail.

Chapter 2 explains in detail how these research objectives were addressed by four individualstudies.

BACKGROUND

High-speed signalized intersections present unique challenges to efforts intended to improve highway safety. Techniques for achieving safety often have an adverse effect on efficiency, and techniques for achieving efficiency sometimes have an adverse effect on safety. For example, efficient operation is achieved when the green phase ends immediately after the queue on the subject intersection approach clears. However, this operation is not always safe because the approach may not be clear at yellow onset, and a driver may be caught in the “dilemma zone.” The dilemma zone is a length of roadway on a signalized intersection approach where drivers as a group demonstrate uncertainty about whether to proceed or stop at the onset of yellow. This uncertainty can lead to rear-end, left-turn opposed, or sideswipe collisions.

Traditionally, engineers have used actuated control with multiple advance detectors to provide safe phase termination at high-speed signalized intersections. Research has shown that systems with this type of advance detection can reduce crashes.⁽¹⁾ However, this advance detection often requires a large gap in traffic before it will allow the phase to end. During high-volume conditions, it is often not possible to find this large gap, and thus, traditional advance detection systems frequently extend the green until the maximum limit is reached (i.e., they max-out). Phase termination by max-out eliminates the desired safety benefit of the advance detection system by abruptly ending the phase, regardless of whether the dilemma zone is occupied. It also suggests that the delay to the minor traffic movements has been lengthy. As a result, the safety and operational benefits provided by traditional advance detection systems diminish as traffic volumes increase.

Bonneson et al. developed an alternative dilemma zone detection and control system for the Texas Department of Transportation (TxDOT).⁽²⁾ The system overcomes the limitations of traditional multiple advance detector systems. The new system, D-CS, intelligently forecasts the best time to end the signal phase based on consideration of vehicle presence in the dilemma zone, vehicle type (i.e., truck or car), and the presence of vehicles waiting for a conflicting phase. At the time this project was getting under way, D-CS had been implemented at eight intersections in Texas and three intersections in Ontario, Canada, and was being planned for other U.S. States.

The functional objectives of D-CS are to both safely and efficiently control the high-speed approaches to an isolated intersection. Safety is measured by D-CS’s ability to reduce crashes related to phase termination (e.g., rear-end crash). Efficiency is measured by D‑CS’s ability to minimize delay to all traffic movements. Bonneson et al. described the manner in which it achieves its functional objectives.⁽²⁾

The next section includes a brief description of D-CS and a status report on its implementation at Texas intersections. The last section describes the findings from a before-after evaluation of D‑CS performance at several Texas intersections.

D-CS

Overview

D-CS is similar to a traditional advance detector system in that it uses information from detectors located upstream of the intersection to extend the green. However, it differs from traditional advance detector systems because it monitors individual vehicles on the intersection approach on a lane-by-lane basis and on a vehicle-length basis. It then uses this information to predict the best time to end the major-road through phase. The D-CS software continuously evaluates and updates this prediction in real time. The prediction is based on the number of vehicles currently in (or predicted to soon arrive in) the dilemma zone as well as the number of conflicting phases with a call for service.

More specifically, D-CS monitors each vehicle on the intersection approach and estimates the number of cars and trucks that are in the dilemma zone at the current time and at every 0.5-s interval for a defined future time interval (typically about 3 s into the future). During a user-specified initial time period (typically about 50 s in duration), D-CS will allow phase termination only when no vehicles are in the dilemma zone. Thereafter, the program concludes that traffic flow is too heavy to find a time when the dilemma zone in every lane is clear, so D-CS seeks the least-cost interval. This cost reflects consideration of the count of vehicles in the dilemma zone for each time interval against the increasing delay that will be incurred by vehicles waiting for service on a conflicting phase. For each interval, it computes a cost of phase termination. The D-CS optimization objective during this second time period is to identify the time interval associated with the least cost. It reassesses this cost matrix every 0.5 s, and, when the least-cost time equals the current time, D-CS ends the phase. The program gives trucks an infinitely high cost so that D-CS is discouraged from ending the phase whenever a truck is in the dilemma zone.

Figure 1 shows D-CS and its relationship to the vehicle detection systems at an intersection. D‑CS consists of a speed trap that is monitored by an enhanced signal controller.^[1] This controller uses the detector output to compute vehicle speed and length. The controller then uses the data to determine the best time to end the phase based on the number and type of vehicles on the major road approach to the intersection, as well as the length of time minor movements have been waiting for service. When the best time to end the phase is determined, the controller ends the phase and transfers service to the next conflicting phase, as defined by the controller ring structure. D-CS uses two detectors in each major-road traffic lane in a speed trap configuration. These detectors are located 800 to 1,000 ft upstream of the intersection on both of the high-speed approaches. (Detector location is flexible in this range and can be adapted to site-specific conditions.)

Source: TTI/Dan Middleton, used with permission.

Figure 1. Illustration. D-CS components.⁽¹⁾

Lane-by-Lane Detection and Vehicle-by-Vehicle Monitoring

A key feature of D-CS is that it can forecast, in real time, when each vehicle in each lane will arrive at and depart from its dilemma zone on the intersection approach. This forecast is based on the D-CS measurement of each vehicle’s speed and time of passage at the upstream detector speed trap. The dilemma zone boundaries are defined in terms of travel time to the stop line (i.e., the zone is defined to begin 6-s travel time from the stop line and end 2 s from the stop line).

The real-time nature of D-CS operation allows it to dynamically accommodate changes in speed that occur at the intersection throughout the day, week, and year. Such changes in speed could be the result of legislated changes in speed limit or the result of changes in traffic density over the course of the day. D-CS performance is not compromised when traffic speeds change, as is the case for traditional advance detection systems. This limitation with traditional systems stems from the fact that their detectors are installed at precise locations that correspond to a specified design speed.

To illustrate the implications of the D-CS dynamic dilemma-zone monitoring process, consider the following example. A vehicle traveling at 70 mi/h is at point A in figure 2, and a vehicle traveling at 25 mi/h is at point B. Neither of these vehicles is in its respective dilemma zone, so D-CS could terminate the phase at this instant in time. In contrast, both vehicles are almost certainly in the zone protected by a traditional multiple advance detector system. The D-CS will correctly end the green interval at this point in time, whereas the traditional system will unnecessarily extend the green interval. This example uses an extreme speed differential to make its point; however, the concept applies to the full range of speeds. The monitoring of individual vehicles, on a lane-by-lane basis, allows D-CS to consistently end the phase sooner than the traditional system. Over time, this capability ensures that D-CS will operate with less delay and catch fewer vehicles in the dilemma zone than the traditional advance detector system.

Source: TTI/Karl Zimmerman, used with permission.

Figure 2. Illustration. D-CS detection design.⁽¹⁾

Previous Implementation Status and Site Characteristics

Previous research by Zimmerman and Bonneson resulted in installation of D-CS at eight intersections in Texas as part of a TxDOT Implementation Project.⁽¹⁾ All implementation sites were isolated, high-speed signalized intersections of a high-volume major road and a low-volume minor road. D-CS is used to control the major-road through movements at each site. Table 1 lists the sites and their characteristics.

The U.S. 84 and Williams Road site was unsignalized prior to D-CS installation. The operational and safety benefits of D-CS could not be separated from those attributed to the addition of signalization, so this site was excluded from the before-after study. Also excluded from the safety evaluation were the two sites at which D-CS was most recently installed (U.S. 84 and F.M.2837 and U.S. 59 and F.M. 3129). These sites were excluded because sufficient time had not elapsed by the date of the report to assess the crash history at these sites during the after period.

A before-after study for each of the five sites designated in bold in table 1 indicated that four had some type of advance detection for green extension prior to the installation of D-CS. The advance detection design varied among locations in terms of the type of detectors used (e.g., loop or video) as well as the number and location of advance detection zones. The site at Loop 340 and F.M. 3400 did not have advance detection prior to the installation of D-CS. This site was deactivated on February 27, 2004, because of nearby construction activity.⁽¹⁾ The intersections included in the before-after study had two through lanes on each approach and a 4- to 4.5-s yellow interval duration. The speed limit varied from 45 to 65 mi/h among the sites.

Table 1. Implementation site characteristics.⁽¹⁾

¹Advance detection used prior to the installation of D-CS.
Loop: multiple advance inductive loop detectors.
VIVDS (video imaging vehicle detection system): multiple advance video detection zones.
Note: Bold indicates sites evaluated in the before-after study.

PREVIOUS EVALUATION OF D-CS PERFORMANCE

This section summarizes the findings and offers conclusions reached from an in-service evaluation of the operational and safety performance of the D-CS. The following measures of effectiveness were used to evaluate its performance:

• Control delay.

• Stop frequency.

• Red-light violation frequency.

• Crash frequency.

The first two measures provide an indication of the operational efficiency of the system. The latter two are an indication of its effect on safety. A decrease in any (or all) of these measures would be an indication of improved conditions as a result of D-CS installation. The evaluation used a before-after study methodology.

This section consists of two subsections. The first subsection summarizes the findings from the analysis of the before-after study data. Additional details of the study design and analysis techniques are available elsewhere.⁽³⁾ The second subsection lists the conclusions based on a review of the findings and experiences with D-CS.⁽¹⁾

Earlier Findings

Table 2 through table 4 summarize the results of the before-after evaluations. The data in table 2 and table 3 were collected during a 4-h before study and a 4-h after study. Table 4 lists the duration of the crash data. As indicated by the data in table 2, intersection operation improved on almost every approach controlled by D-CS. The increase in delays and stops on the southbound approach of U.S. 281 and Borgfeld Road is believed to be due to the significant increase in minor movement traffic volume that was observed in the after period at this site. Overall, D-CS reduced control delay by 14 percent and stop frequency by 9percent. These reductions are likely the result of the more efficient operation of D-CS relative to the detection and control strategy that was in operation prior to the D-CS installation.

The data in table 3 indicate that the frequency of red-light violations was reduced on all but one approach controlled by D-CS. The increase at this one location was not statistically significant and was rationalized to be a result of random variation in the data. Overall, violations dropped by 58 percent, and violations by truck drivers dropped by about 80 percent. When D-CS replaced an existing multiple advance loop detection system, violations dropped by 53 percent. When D-CS was installed at an intersection that did not previously have advance detection (i.e., Loop 340 at F.M. 3400), violations declined by about 90 percent.

Table 2. Before-after delay and stop frequency comparison.⁽¹⁾

¹Relative change = (after/before -1) x 100.
²Negative values denote a reduction.
*Values are statistically significant at the 95-percent level of confidence.

The data in table 4 indicate that the frequency of crashes dropped at all of the intersections at which D-CS was installed. Overall, there was a 39-percent reduction in severe crashes on the twoapproaches controlled by D-CS. The data suggest that 9 severe crashes (and about 18 property-damage-only crashes) were prevented in the time that D-CS was in operation. If only those crashes that are influenced by D-CS are considered (i.e., rear-end, left-turn opposed, and sideswipe), then D-CS installation accounted for a 50-percent reduction in severe “influenced” crashes.⁽¹⁾

Conclusions Based on Earlier Installations

The objective of the D-CS is to safely control the major-road approaches to an isolated, signalized intersection without creating excessive delay to minor movements. This objective was achieved by developing a system with the following benefits (relative to the traditional multiple advance detector system):

• Reduces the frequency of red-light violations.

• Reduces the frequency of crashes associated with the phase change (e.g., rear-end crashes).

• Reduces delay and stop frequency on the major road.

• Maintains or reduces overall intersection delay.

Table 3. Before-after red-light violation comparison.⁽¹⁾

¹Frequency of red-light violations during study (study duration for each approach listed in table 4).
²Relative change = (Obs. After/Exp. After -1) x 100. Negative values of relative change indicate a reduction in violation frequency.
*Values are statistically significant at the 95-percent level of confidence.

Table 4. Before-after severe crash frequency comparison.⁽¹⁾

¹Relative change = (Obs. After/Exp. After -1) x 100. Negative values of relative change indicate a reduction in crash frequency.
*Values are statistically significant at the 95-percent level of confidence.

The first two benefits are realized by predicting the time every driver is in his or her dilemma zone and by searching for a time in the near future when the total number of drivers in their respective dilemma zones is at a minimum. This future time is defined as the “best time to end the phase.” In short, D-CS is a dynamic dilemma-zone monitoring system that identifies the dilemma zone for each vehicle in real time and prior to when the information is needed for signal control decisions. D-CS operation differs from that of a multiple advance detector system because the latter system searches for a time when a segment of each approach is clear of vehicles.⁽¹⁾

The last two benefits identified in the bullet list are realized in two ways. First, they are partly achieved by the D-CS algorithm’s dynamic dilemma-zone monitoring process. This process is often able to find the “best time to end the phase” sooner than a multiple advance detector system. This capability translates into shorter phases and lower overall delay. Second, D-CS does not allow the stop-line detector to extend the phase once the queue has been served. This feature reduces wasted green time at the end of the phase and minimizes delay to waiting vehicles. These benefits are most evident at higher flow rates.

D-CS provides additional safety benefits for trucks. D-CS has the ability to measure the length of approaching vehicles and, using this information, to postpone phase termination whenever long vehicles are in the dilemma zone. Multiple advance detector systems do not provide this sensitivity.⁽¹⁾

^[1]The D-CS enhanced controller is manufactured by Naztec (as part of Trafficware).