Signing Two-Lane Freeway Exits with an Option Through Lane in Extreme Conditions: Anatomy of Drivers’ Behavior

Abstract

Two-lane freeway exits in which the left-hand exit lane also continues as a through (option) lane are becoming increasingly common. Potential problems can arise at these exits when drivers become confused by the signing and can lead to increases in driver frustration, incidents, and congestion. These problems have not been studied in any detail, especially in circumstances in which the signs recommended in the Manual on Uniform Traffic Control Devices for Streets and Highways cannot be used. Such is the case in the underground sections of I-93 through downtown Boston, Massachusetts, where the signs can be no more than 3 ft high. To arrive at a better understanding of these problems, the Massachusetts Highway Department evaluated a number of different ways of informing drivers that the left-hand lane of a two-lane exit functioned as both an exit lane and a through lane. The entire underground sections of northbound and southbound I-93 were simulated on a computer. Participants in an experiment sat in an actual car surrounded by three screens onto which was projected the virtual world and manipulated the controls of the car just as they would those of an ordinary vehicle. The participants were given a particular destination on each drive. The exact path was recorded, and misses and unnecessary lane changes could be derived from the recording. Six different signs were evaluated in three experiments. A function that could be used to rank the effectiveness of a given sign on the basis of the participants’ different responses to the sign was developed. Clear recommendations for the signing of two-lane freeway exits with an option through lane in extreme conditions emerged from the experiments.