Nicholas J Garber

Estimating truck accident rate and involvements using linear and poisson regression models

During the past few years, vehicle miles of travel (VMT) for large trucks having six or more wheels in contact with the road and having a gross weight greater than 10,000 lbs, have been steadily increasing. This has resulted in an increased interaction between large trucks and other vehicles which is being manifested in an increasing rate of fatal accidents involving large trucks. For example, between 1982 and 1984, fatal accident rates for 100 million VMT for large trucks in Virginia, increased by 54% while the rate for other vehicles (passenger cars, vans and pickups) remained approximately constant at less than a 0.3% increase. In order to arrest this increasing trend in large truck fatal accident rates, it is necessary that appropriate counter measures be developed. For this to be done, however, it is necessary to identify those factors that are associated with large truck accidents. A recent analysis of large truck accident data in Virginia, indicated that certain traffic and highway geometric charac...

Effect of Speed, Flow, and Geometric Characteristics on Crash Frequency for Two-Lane Highways

Although during the past several years significant progress has been made in improving safety on U.S. highways, the frequency and severity of crashes continue to be of concern. A better understanding of the factors associated with crashes will facilitate the identification of suitable countermeasures that could further reduce the occurrence of crashes. Results are presented from a study that determines how the characteristics of speed, flow, and geometry affect the crash rates for two-lane highways. Deterministic models that relate the crash rate with the mean speed, standard deviation of speed, flow per lane, lane width, and shoulder width are presented. The multivariate ratio of polynomials method was used to develop the models. Research was limited to two-lane roadways in Virginia with speed limits of 89 km/h (55 mph). The data were obtained from speed-monitoring stations established by the Virginia Department of Transportation and from police accident reports from January 1993 to September 1995.

Distribution and Characteristics of Crashes at Different Work Zone Locations in Virginia

Work zones tend to cause hazardous conditions for drivers and construction workers, because they create conflicts between construction activities and traffic, thus exacerbating the existing traffic conditions. Every effort should therefore be made to minimize the negative impacts of work zones. A clear understanding of work-zone crash characteristics will help to determine appropriate measures to minimize work-zone hazards. This study investigated the characteristics of work-zone crashes that occurred in Virginia from 1996 through 1999. The information on each crash was obtained from police crash records. Each crash was located in one of five areas of the work zone—(a) advance warning, (b) transition, (c) longitudinal buffer, (d) activity, and (e) termination. The percentage distributions were analyzed relative to crash location, crash severity, collision type, and highway type. The proportionality test was used to determine significant differences at the 5% significance level. The results indicate that the activity area is the predominant location of work-zone crashes regardless of highway type, and rear-end crashes are the predominant crash type. The results also indicate that the proportion of sideswipe-in-same-direction crashes in the transition area is significantly higher than that in the advance warning area.

Safety Effects of Differential Speed Limits on Rural Interstate Highways

To compare the safety effects of a uniform speed limit (USL) for all vehicles and a differential speed limit (DSL) for cars and heavy trucks, crash, speed, and volume data for rural Interstate highways for the period 1991 through 2000 were obtained from nine states. These states were divided into four policy groups based on the type of speed limit employed during the period: maintenance of a uniform limit only, maintenance of a differential limit only, a change from a uniform to a differential limit, and a change from a differential to a uniform limit. Statistical tests (analysis of variance, Tukey’s test, and Dunnett’s test) and the empirical Bayes method were used to study speed and crash rate changes in the four policy groups. The focus here is only on the statistical tests. The safety effects of DSL and USL were not different within the scope of the study. The mean speed, 85th percentile speed, median speed, and crash rates tended to increase over the 10-year period regardless of which type of speed limit was employed. When all sites within a state were analyzed, temporal differences in these variables were often not significant.

Calibrating Pedestrian Level-of-Service Metrics with 3-D Visualization

Unlike the case with airport terminals or the central business district, the quality of suburban pedestrian facilities is most likely affected less by congestion and more by safety, the walking environment, and aesthetics. Because the Highway Capacity Manual does not explicitly capture such factors when measuring pedestrian level of service (LOS), researchers have proposed innovative rating scales that do. These scales use either measurable characteristics, such as walkway width, median openings, and signalization parameters, or user perceptions, such as continuity and convenience, to rate a pedestrian facility. Unfortunately, the results of these scales are not always easy to interpret. For example, in a scaling system for pedestrian facilities in which a raised curb median counts 6 points and a blinking pedestrian-crossing signal counts 3 points, the developers of the scale believed that the median would be twice as valuable to pedestrians as the crossing signal. But would pedestrians agree? A scaling system was developed for pedestrian LOS and calibrated using visualization (computer-aided modeling techniques consisting of still shots and animations). Subjects’ perceived ratings of a pedestrian facility after they viewed still pictures and animations of the facility were compared with the computed rating of the facility from an LOS scale. The chief value of this method is that it helps ensure that pedestrian crossing needs are systematically considered and that engineers, planners, and the public agree on the calibration of a pedestrian LOS scale. The methodology is also applicable in urban areas where pedestrian needs beyond physical capacity are to be explicitly considered. The approach is original in that visualization as a simulation and data analysis tool was used to calibrate a pedestrian LOS scale.

Influence of exposure duration on the effectiveness of changeable-message signs in controlling vehicle speeds at work zones

In work zones, drivers do not slow down in response to conventional signs. Changeable-message signs (CMSs) equipped with a radar unit can be used to display specific warning messages to speeding drivers. The radar unit detects the speed of each vehicle entering the work zone and can be programmed to activate the CMS if the speed of the vehicle exceeds a preset threshold value. A two-phase longitudinal study was conducted. The first phase, conducted by Garber and Patel, examined the short-term effectiveness of the CMS with radar in reducing vehicle speeds in work zones. The second phase, some of the results of which are presented, evaluated the influence of the duration of exposure of the CMS with radar on its effectiveness in reducing speeds in work zones. Speed and volume data for the population were collected at the study sites by automatic traffic counters placed at the beginning, middle, and end of each work zone. In addition, the speeds of individual drivers who triggered the CMS by exceeding the threshold speed were also recorded (using a video camera) at two other locations within the work zone for several weeks and analyzed. The results of the study indicated that the duration of exposure of the CMS does not have a significant impact on speed characteristics and driver behavior. Therefore, the CMS continues to be effective in controlling speeds in work zones for projects of long duration. The results also indicated that the CMS with radar reduces the probability of speeding at work zones and that this effect is maintained for at least up to 7 weeks.

Incorporating Crash Risk in Selecting Congestion-Mitigation Strategies: Hampton Roads Area (Virginia) Case Study

The extensive research under way to forecast traffic volumes from realtime data is expected to result in a significant increase in the use of realtime data for congestion mitigation and management in the next few years. Unfortunately, congestion-mitigation strategies based solely on these forecast volumes will disregard the impact of other traffic variables on crash occurrence. This may result in congestion-mitigation strategies that improve traffic flow but significantly increase crash rates. These other variables, for which real-time data are also available in traffic management centers, include speed and occupancy. However, real-time data have not been widely used in research to develop mathematical models that relate crash rates with these other traffic variables. The availability of this type of relationship will enable engineers in the field to develop congestion-mitigation strategies based not only on forecast volumes but also on real-time information on traffic variables such as speed and occupancy. A procedure that uses real-time data on traffic flow, speed, and occupancy and the relationship between these variables and crash occurrence could be used to develop congestion-mitigation strategies that incorporate safety. Currently, no such decision-support tool is readily available to the traffic engineer. The feasibility of developing such a methodology in which real-time data could be used to develop congestion-mitigation strategies that also consider crash risk was investigated. Models showing the relationship between flow and occupancy and between number of crashes and occupancy were developed for specific sites in the Hampton Roads, Virginia, area. These models were then used to illustrate how diversion strategies that consider crash risk may be developed.

Safety Impacts of Photo-Red Enforcement at Suburban Signalized Intersections an Empirical Bayes Approach

Photo-red enforcement entails cameras that photograph a vehicle entering an intersection after the traffic signal has turned red; a citation is then mailed to the vehicles owner. Such a program was operated at 13 signals in Fairfax County, Virginia, between 2000 and 2005. This paper reports how photo-red enforcement affected crash frequency at the intersections. An empirical Bayes approach was used to detect the impact of the program on crashes while main-line traffic volume, yellow time, truck percentage, number of left-turning lanes, and speed limit were controlled. Cameras were definitely correlated with decreased red light running crashes (11% to 55%), increased rear-end crashes (32% to 85%), increased total crashes (0% to 23%), and decreased red light running injury crashes (5% to 64%). Cameras were probably associated with increased total injury crashes (5% drop to a 33% raise). These results are consistent with those of other studies with one notable exception: In the case of Fairfax County, the m...

Guidelines for Using Late Merge Traffic Control in Work Zones: Results of a Simulation-Based Study

The late merge traffic control strategy has been proposed as a way to improve flow at work zone lane closures by maximizing queue storage space and creating more orderly merging. The late merge instructs drivers to use all lanes to the work zone taper and then take turns proceeding through the work zone. There is little information available on when the late merge should be used, however, and a limited understanding of the factors that influence its performance. This paper discusses the results of a simulation study of the late merge concept using microscopic traffic simulation. The late merge concept was evaluated by comparing it with traditional traffic control, through a full factorial analysis. Results of the computer simulations showed that the late merge produced a statistically significant increase in throughput volume versus the traditional merge for the three-to-one lane closure configuration across all combinations of analysis factors. Although the two-to-one and three-to-two configurations did not show significant improvement in throughput overall, it was found that as the percentage of heavy vehicles increased, the late merge did foster higher throughput volumes than traditional traffic control. The results of the simulations indicate that the late merge may not provide as much of a benefit as previous studies had indicated and that the area of application for the late merge may be limited to situations where heavy vehicles constitute more than 20% of the traffic stream.

Traffic signal timing at isolated intersections using simulation optimization

Two innovative stochastic traffic signal optimization techniques for isolated intersections are discussed. The objective is to determine the optimum cycle and green phase lengths for signalized isolated traffic intersections. Determination of optimum cycle and green phase lengths is based on minimization of the total average delay at the intersection for a given period of observation. Traffic signal timing is formulated as a stochastic inventory problem, which is then solved by a combination of simulation and dynamic programming. The suitability of the optimization techniques for undersaturated and oversaturated flow conditions; and steady state and nonstationary queue conditions are discussed. The advantages of the techniques over most widely used signal optimization methods, and the application of the simulation optimization method in state dependent server-vacation signal timing are also discussed.