Development of Access Spacing Guidelines for Nonfreeway Weaving Environments

Access on the frontage road in close proximity to exit ramp terminals can amplify the amount and severity of weaving that occurs and can lead to operational and safety problems on the frontage road. Research activities directed at evaluating the operation of frontage roads with unsignalized marginal access located at different distances from exit ramp terminal points and developing guidelines for appropriate spacing under these conditions are summarized here. The basic research approach consisted of (a) analyzing accident data; (b) observing operations in the field to identify distances required to safely make weaving maneuvers; and (c) developing an analytical model to predict the density of the weaving section on the frontage road as a function of frontage road volume, exit ramp volume, total driveway volume, frontage road configuration, and exit ramp to access spacing. The model was developed from the results of a computer simulation (using CORSIM) that was calibrated with field data from several frontage road sites in Texas. Results of the accident and weaving (field observation) analyses were used to develop a recommended minimum distance of 140 m between exit ramp terminal points and the nearest frontage road access; the analytical model was used to develop desirable spacing distances ranging from 140 to 300 m. The model was also used to identify possible level-of-service boundaries that can be used to assess the quality of service provided on a particular section of frontage road.