Evaluation of Ground Mounted Diagrammatic Entrance
Ramp Approach Signs
Authors:
October 2000
State Job No. 14674(0)
Abstract
Highway-freeway interchanges can be a source of confusion to
unfamiliar drivers. Typically, freeway entrance ramp information is provided by
means of trailblazer assemblies (interstate or highway shield, cardinal
direction, and a small white arrow on a blue shield). Trailblazer assemblies
are generally located right at the interchange location, giving unfamiliar
drivers very little advance notification as to the proper lane choice needed to
gain access to the freeway entrance. Advance located ground mounted
diagrammatic guide signs provide this much needed information well in advance
of the interchange entrance, thus giving drivers more time to change lanes, if
needed. Diagrammatic signs have been used in
Six highway-freeway interchanges were selected in the Greater Columbus, Ohio, area to determine the effectiveness of the diagrammatic signs in the field. Traffic flow video footage was collected at the selected sites before the diagrammatic signs were installed. The finalized diagrammatic sign designs were then installed and the traffic flow videotaping was repeated. The collected video footage was automatically analyzed with Mobilizer PC. The resulting vehicle count, speed, and headway data was used as input to a closed form analytical model to determine the probability that drivers could successfully change lanes as needed, prior to reaching the gore of the freeway entrance. The theoretical analysis indicated a much higher probability that an unfamiliar driver could execute a required lane change when diagrammatic signs are used. ODOT/FHWA evaluators visited the six interchange sites and provided their input and opinions as to the use of diagrammatic signs. The vast majority of the evaluators fully embraced the idea of diagrammatic signs. Eye movement recordings were performed at night at the six sites in order to determine if the presence of the diagrammatic signs was distracting. The results of the eye movement analysis indicate that the diagrammatic signs are not looked at excessively often or excessively long. The overall median first look distance to the diagrammatic signs was found to be 125m. An evaluation with unfamiliar test drivers corroborated the findings of the theoretical model analysis, in that drivers were able to execute a needed lane change much earlier when diagrammatic signs were provided. A set of application guidelines for diagrammatic signs is provided.