| Freeways have a distinctive characteristic of offering the highest level of mobility with full control of access. Access to freeways is provided through interchanges with a system of loops and ramps. Adequate lengths between ramp terminals, with enough length for speed change lanes, give the road users enough time to find acceptable gaps, accelerate, decelerate, and merge or diverge properly. Relatively short spacing forces drivers to perform unsafe manoeuvres and hence increase the probability of collisions. Most of previous research has focused on analyzing the operational conditions of the merge and diverge areas using a speed-based analysis. However, little research has handled the effect of merging and diverging from a safety-explicit point of view. Twenty-six interchanges along Highway 417 within the City of Ottawa were selected to quantify the effects of ramp terminal spacing and traffic volumes on safety performance through regression analysis. Poisson and negative binomial models were developed using collision data covering five years from 1998 to 2002. Modelling attempts resulted in 23 statistically significant models relating factors such as traffic volumes and geometric features to collision frequency. |
