There is a significant need to explicitly include highway safety in the highway design process, in that safety has so far often been generally limited to qualitative assessments of design alternatives. To receive proper consideration, safety needs to be dealt with quantitatively. Unfortunately, highway safety is seldom, if ever, explicitly considered during the highway design process. This omission can be attributed to various factors, the most important one being the lack of available tools needed for estimating the number of crashes during this process. To help fill this void, the objective of this study is to develop a tool that would allow the estimation of crashes for four types of ramp design configuration. The tool proposed in this work will be useful during the project planning process where it can be implemented to identify cost-effective interchange ramp design configurations. Only ramps in non-frontage-road settings were analyzed for this study. The four basic ramp design configurations analyzed were: 1) diagonal ramp, 2) non-free-flow loop ramp, 3) free-flow loop ramp, and 4) outer connection ramp. Predictive models were proposed for the four ramp design configurations. Predictive models, developed from data collected in Washington for the years 1993-1995 inclusively, were calibrated using data from Texas. Crash data, traffic flows and geometric design characteristics were collected at 31 ramps for the years 1998 to 2000. Calibration factors were estimated using a procedure recently proposed in the Interactive Highway Safety Design Model (IHSDM). An example detailing the application of the tool for the design of ramp configurations is presented in this study.
Samenvatting