David Levinson, Professor, Civil, Environmental and Geo-Engineering
Although the ramp metering system may reduce total drive time, it may not reduce drivers' total perceived travel time. If time at the ramp is not weighted the same as time-in-motion by users, this time-minimizing strategy may not be utility-maximizing for travelers. This research attempted to quantify the weights individuals associate with qualitatively different experiences of travel time: waiting at a ramp meter or freeway-to-freeway ramp meter and traveling at different freeway speeds requiring varying numbers of acceleration and deceleration shifts. We employed two methodologies to study individual preferences for ramp and traffic flow conditions: (1) The Computer Administered Stated Preference method (CASO) describes alternative scenarios, then asks drivers to indicate their preferences; (2) The Virtual Experience Stated Preferences (VESP) utilizes a simulator where drivers experience a virtual simulation of alternative scenarios, then evaluate them. Different scenarios were produced by varying the wait time at ramp meters and the resultant speeds at which drivers were able to drive on the freeway. The goal was to use driver preferences to recommend ramp meter waiting times that correspond to their positive perceptions.
In both methodologies, the subjects were asked to rate each scenario and to rank them in order of preferences. By comparing the ratings and rankings of one method to another, we hoped to test the hypothesis that CASP and VESP would produce similar results. The preference data from CASP methodology did not mirror the data obtained with VESP methodology. The lack of agreement suggests there are problems with one or both methodologies and additional research is needed to understand how stated preference techniques are interpreted by subjects. Experiments employing real cars on real streets would enable us to corroborate or refute the results from computer-based or driving-simulator-based stated preference experiments.