Possible Expectancy Violations on Frontage and Conventional Roads at Interchanges

The intersection and mandatory movement lane control signs placed on intersection approaches are critical to safe and efficient intersection operations. Ramp, frontage road, and cross-street approaches to interchanges often widen at intersections to accommodate additional through or turn lanes. Currently, there is inconsistency in conveying to drivers how they should align themselves upstream of a diamond intersection to maneuver for their desired turning movement as the intersection widens. These inconsistencies can result in drivers making an incorrect lane selection that may result in late lane changes or illegal turns. This paper focuses on the expectancy violations that were discovered by a driver survey portion of a larger project that included practitioner surveys and a field evaluation of sign alternatives. A computer-based driver survey sampled 204 Texas drivers in four cities. The questions reported in this paper focused on driver expectations of lane movement and assignment on frontage road approaches to cross streets on freeway on- and off-ramps. Findings that showed that drivers are often incorrect in their assumptions about lane assignment when approaching intersections indicate a greater need for advanced lane control signs. A new sign design, adopted from Australian practice, that graphically shows lane additions and drops was the most effective for cases in which the downstream geometry varied from the driver's expectations.