The significance of the posted minimum speed limits along interstate highways in South Carolina on traffic operation and safety

This paper evaluated the significance of the posted differential speed limits(DSL)on traffic safety and operation along interstate highways in South Carolina.This paper defines DSL as the difference between the posted maximum and minimum speed limits.The paper used vehicle speed and traffic crashes for analysis.The evaluation results showed over 99%of all vehicles complied with the posted 45 mph minimum speed limit and more than 50%of drivers drove above the posted maximum speed limit regardless of the posted maximum speed limits.The results also indicated that speed variations increased as the DSL increased.However,the safety evaluation showed conflicting results on the impacts of the DSL on safety.The results showed the DSL had negative implications on crashes per AADT per lane.This is attributed to increased interactions between slower and faster-moving vehicles in the traffic stream.On the contrary,it was observed that segments with higher DSL had fewer crashes per mile per lane than the segments with the lower DSL.These results suggest there is a need to conduct more research to quantify the efficacy of posting the minimum speed limit because there were no problems observed with motorists driving too slowly on interstate highways.The results provide important information on the efficacy of posting minimum speed limits to improve the uniformity of traffic flow and safety.

A methodology for calibration of traffic micro-simulator for urban heterogeneous traffic operations

Traffic micro-simulation is a widely accepted tool in many countries for the evaluation and assessment of alternative design schemes.However,for several developing countries,replicating heterogeneous,non-lane based traffic in a micro-simulation framework is gaining increased importance and still remains a challenge due to its complexity.The present study demonstrates a methodology to calibrate a traffic micro-simulation model giving due consideration to vehicle-class specific driver behavior in an urban Indian scenario for a midblock section and an intersection approach in Kolkata.The sensitive parameters affecting the driver behavior were identified for every vehicle type using Latin Hyper cube design,taking vehicle class specific travel time as a performance measure.Linear regression models were developed for each vehicle class considering the sensitive driving behavior parameters.The models highlight that the dependency of measure of effectiveness(MOE)of one vehicle type is not only limited to its own driver behavior but also on parameters of other vehicle classes.A genetic algorithm based optimization was adopted to obtain optimal parameter sets for different vehicle classes.The optimum values were found to vary significantly across all vehicle classes at a 95%confidence level.Single and multi-criteria calibration principles are also implemented to yield much more realistic results and subsequently minimizing weighted error for all vehicle classes.

Analyses of maximum-speed path definition at single-lane roundabouts

The process of designing roundabouts is an iterative process through which, in several checks, the design elements of a roundabout get optimized. Existing regulations for roundabouts involve swept path analyses, sight distance analyses and speed analyses of vehicles passing through the roundabout. Speed analyses are done mostly based on two models, Dutch and American. Each of these two models, in their own way takes into account design elements of the roundabouts, and the US model also envisions the construction of vehicle paths through the roundabout. Main assumption of both models is that vehicle paths through roundabouts consist of few connected radii. US models for path definition takes into account safety distances from marked lines and geometric elements（curbs） at the entrance and exit and through roundabout. Experimentally determined elements of the vehicle path through the roundabout, do not correspond to those recommendations. Comparison of the measured speed at the roundabouts and speed calculated according to aforementioned models at several roundabouts in Croatia, showed a significant difference. An experimental research was conducted as a first step in developing a new model for operating speed through roundabouts. The research aimed to define the basic path elements of vehicle movement in the roundabout at which the maximum speed is achieved. Results of the study are presented in this paper.