Bhagwant Persaud

Calibration and transferability of accident prediction models for urban intersections

Accident prediction models, also known as safety performance functions, have several important uses in modern-day safety analysis. Unfortunately, calibration of these models is not straightforward. A research effort was undertaken that demonstrates the complexity of calibrating these models for urban intersections. These complexities relate to the specification of the functional form, the accommodation of the peculiarities of accident data, and the transferability of models to other jurisdictions. Toronto data were used to estimate models for three- and four-legged signalized and unsignalized intersections. Then the performance of these models was compared with that of models for Vancouver and California that were recalibrated for Toronto using a procedure recently proposed for the application in the Interactive Highway Safety Design Model (IHSDM). The results of this transferability test are mixed, suggesting that a single calibration factor as is currently specified in the IHSDM procedure may be inappropriate and that a disaggregation by traffic volume might be preferable.

Development of a procedure for estimating expected safety effects of a contemplated traffic signal installation

The Manual on Uniform Traffic Control Devices contains warrants for traffic signal installation but cautions that satisfying a warrant does not in itself justify the decision to install a signal and that one should not be installed unless an engineering study indicates that this will improve the overall safety or operation of the intersection. The development of an easily implementable procedure, which is intended to be part of an engineering study, for estimation of the expected safety effects of a contemplated signal installation is reported. These effects can then be considered in conjunction with other impacts in a conventional economic evaluation. The development of the procedure by use of a multijurisdiction database is described, and a detailed illustration is presented. Use is made of the empirical Bayes methodology that of late has been recognized as the state of the art in safety estimation and of the most recent advances in that methodology. Substantial focus is placed on the application of that methodology and on the development of the accident prediction models required to support that application. The development of the procedure is part of an NCHRP project (NCHRP 17-16) that aims to improve the safety warrant for signal installation and, more generally, to determine how safety is considered in the decision to install or not install a signal.