The Effects of Lead-Vehicle Size on Driver Following Behavior: Is Ignorance Truly Bliss?

The objective of this study was to examine whether size of a lead vehicle (passenger car or light truck) affects the distance at which following vehicles travel. Naturalistic following data were collected from drivers using instrumented passenger cars in place of their own vehicles. The results show that these drivers followed light trucks at shorter distances than they followed other passenger cars by an average of 5.6 m, or .19 s in headway time margin, but at the same velocities and range-rates. This result is discussed in the context of a passenger car driver's ability to see beyond a lead vehicle to assess, and respond to, the status of traffic downstream. The results of this study suggest that knowing the state of traffic beyond the lead vehicle, even by only one additional vehicle, affects gap length. Specifically, it appears that when dimensions of lead vehicles permit other drivers to see through, over, or around them, drivers maintain significantly longer (i.e., safer) distances.