Francis P. D. Navin

SIMULATION OF TRAFFIC CONFLICTS AT UNSIGNALIZED INTERSECTIONS WITH TSC-SIM

This paper describes a traffic conflicts computer simulation model and graphic display for both T and 4-leg unsignalized intersections. The goal of the model is to study traffic conflicts as critical-event traffic situations and the effect of driver and traffic parameters on the occurrence of conflicts. The analysis extends conventional gap acceptance criteria to describe drivers behaviour at unsignalized intersections by combining some aspects of gap acceptance criteria and the effect of several parameters including drivers characteristics such as age, sex, and waiting time. The effect of different traffic parameters such as volume and speed on the number and severity of traffic conflicts is also investigated. The model is unique insofar as it uses a technique of importance sampling and stores the traffic conflicts that occur during the simulation for later study. A graphical animation display is used to show how these conflicts occurred and the values of critical variables at the time. Model results were evaluated against previous work in the literature and validated by using field observations from four unsignalized intersections. The simulation results correlated reasonably well with actual conflict observations and should prove useful for assessing safety performance and feasible solutions for other unsignalized intersections.

Automobiles on Horizontal Curves: Experiments and Observations

Statistical information on the basic variables involved in driving through a horizontal curve was obtained using a 4×4 Latin square design experiment to measure the action of automobile drivers in test track horizontal curves. The independent variables used in the test curves were speed (comfortable, fast); pavement surface (dry, wet); driver (male, female); and curve radius (16 m, 26 m, 60 m, 100 m). The measured output was the driver’s selected speed and corresponding lateral acceleration. In addition, the passengers indicated their comfort level on a four-point semantic scale. Expert drivers also drove the test curves to establish the upper limits of the driver-vehicle-tire system. Field observations of four curves along a two-lane rural mountain highway measured driver vehicle speed, lateral acceleration, and lateral position. The results indicate that, for a comfortable ride, drivers are limited by their comfortable lateral acceleration on small radius curves and seek the “environmental speed” on large radius curves.

VEHICLE TRACTION EXPERIMENTS ON SNOW AND ICE

Traction tests were run during February, 1993 and 1994. The snow tests were conducted at a fairly constant temperature of -2 degrees C, and the ice tests at air temperatures ranging from -4 to -35 degrees C. The test vehicles were a standard midsize automobile and highway maintenance gravel trucks. The automobiles on packed snow at -6 degrees C have an average braking force coefficient of 0.35, a lateral force coefficient of 0.38, and a traction force coefficient of 0.20. The corresponding values for a straight truck are: 0.23, 0.35, and 0.15. An automobile on bare ice at -6 degrees C has an average braking force coefficient and lateral force coefficient of 0.09, and a traction force coefficient of about 0.08. The valves for the truck on bare ice in the same order are 0.06, 0.07, and about 0.04. A relationship was developed between the average braking force coefficient, ambient temperature, and the amount of standard highway winter aggregate used on the road. The influence of winter aggregate on ice gave an improvement of about 15% at a highway maintenance light application of 75 gm/m2, and about 30% at a heavy application rate of 150 gm/m2. Aggregate blow off tests on bare pavement estimated that at an application rate of 175 gm/m2 about 100 cars or 30 trucks would remove most of the winter aggregate from a bare road surface. (A) For the covering abstract of the conference see IRRD 898597.

Improving Traffic Safety: A New Systems Approach

A guiding principle of modern traffic safety professionals attempting to reduce the risks associated with traffic is to holistically address traffic safety as a multidisciplinary partnership issue. The systems approach focuses on the relationships and dependencies between the various elements of the traffic system. The C3-R3 Systems Approach to traffic safety is introduced; the building blocks of the C3-R3 approach are three entities (the road user, the vehicle, and the road environment), three pre-crash timeline phases (creation, cultivation, and conduct), and three postcrash timeline phases (response, recovery, and reflection). This approach is proposed as a framework for multidisciplinary traffic safety professionals to research traffic safety issues in an integrated, systematic manner. The C3-R3 approach provides an enhanced systematic framework that more clearly identifies the stages at which traffic safety professionals can intervene to promote road safety. The graphical representation of the C3-R3 system, as presented, emphasizes the convergence of the entities as the timeline proceeds toward a crash event and their subsequent redivergence in the postcrash timeline. Every combination of entity and timeline phase represents a cell in the C3-R3 system; the contents of each cell represent the individual elements that traffic safety professionals need to focus on and understand in order to reduce the crash risk. The C3-R3 Systems Approach represents a starting point to encapsulate the systems approach concepts in traffic safety. It is expected that as more professionals adopt systems thinking, the C3-R3 approach will continue to evolve, expand, and improve.

Road Safety Model: Some Fundamental Ideas

A fundamental relationship has been developed that explains road accident statistics in developed and developing countries. The model uses two variables, traffic hazard measured as deaths per vehicle and motorization measured as vehicles per person, to estimate personal hazard as deaths per person. Special cases of the model are those by Smeed, Trinca et al., and Koornstra. The model of fatalities has two extremes. Early motorization has high traffic hazard and personal safety is low and increasing. Full motorization is characterized by a moderate and falling traffic hazard and a low and decreasing personal safety. Between these extremes, there is a maximum number of fatalities per population. Models for personal injury and total road accidents in developed countries appear to follow a similar trend. Available world data fit the proposed relationships well. The models allow planners and engineers to estimate the future maximum road fatalities for developing countries. The model has been extended to incorporate an automobile ownership model that explains some of the growth in motorization. A traffic hazard model is also outlined, in part on the basis of the ideas developed by Koornstra. The extended models should allow a more detailed analysis of some of the social and engineering factors that contribute to road safety.

RECONSTRUCTION OF ACCIDENTS INVOLVING HIGHWAY BARRIERS

The full scale crash tests on which this report is based focused on the performance of a segmented concrete barrier used along the highways of British Columbia. The barriers are similar to those used along many construction zones of the U.S. The opportunity was taken during a number of the tests to make a detailed record of the vehicle damage. The purpose of these vehicle damage measurements was to investigate how well vehicle damage could be related to the original test impact conditions. The main pre-impact condition of interest in many situations is speed.

Safety of Roadside Curbs

There has been considerable experimental research of cars crashing into roadside curbs. This paper uses all the published information to establish a relationship to estimate the ability of a curb to safely redirect a vehicle. A curbs ability to redirect a vehicle depends upon the speed and angle of impact, the surface material, if it is wet or dry and the radius of the impacting tire. There are other factors such as the aggressiveness of the tire tread and the tire pressure that are thought to be important but have not been incorporated into the analytical procedure. The equations may be used in accident reconstruction to estimate a minimum vehicles speed to mount a curb. (A) For the covering abstract of the conference see IRRD 899758.

HYDROPLANING AND ACCIDENT RECONSTRUCTION

Automobile hydroplaning speed is affected by both the vehicle load on the tyre and its inflation pressure, yet only inflation pressure is valued in Hornes (1968) equation. In 1984 modifications were made to include a vehicles tyre footprint characteristics. Dunlop and others (1974) studied the influence of water depth and tread depth on an automobiles hydroplaning speed. Empirical studies by Gallaway and others (1979) produced more conclusive hydroplaning speeds for both automobiles and Ivey and others (1984) for trucks. This paper uses an influence diagram to show how all the models are related. Using the model a few vehicle design parameters are pursued that may be combined to make vehicles more prone to hydroplaning. For the covering abstract of the conference see IRRD 882390.

Statistics of Extremes: An Alternate Method with Application to Bridge Design Codes

A method is developed for determining upper bounds for the quantiles of the distribution of the maximum of a fixed or random number of positive variates which are positively orthant dependent. (Such bounds are of practical interest e.g. as in setting the design codes of structures which must sustain random loads.) The bounds obtained using the method will be quite conservative unless the variates themselves are bounded. If they are bounded and the number of variates is fixed and large and/or the quantile desired is extremely high, the bound will approximate the true quantile of the Type 111 limiting distribution obtained by Gumbel [5]. Unlike Gumbels asymptotic result, this method may be applied when the number of variates is small and so avoids some ambiguity about its applicability. On the other hand it is more complicated since it involves an application of a basic inequality for random variables.

Model for Road Safety Planning: Theory and Policy Example

A fundamental relationship has been developed that should allow authorities to forecast the results of various road safety interventions. Risk is defined to include attributes of the road, vehicle, driver, and speed enforcement. The model may be linked to the traditional Forgiving Highway design and to vehicle crashworthiness. The probability of a crash, or the Caring Highway variables, may also be studied for policy decisions. Also, by manipulating the exposure element, the model may be used by transportation planners to include road safety in their analysis of transportation impacts. The new model will help authorities—in developed and developing countries—to select the most effective road safety programs and to understand some of the interactions between programs.