Two’s a Crowd: Factors Affecting Time-to-Collision Estimations of Multiple Approaching Vehicles

Accurate time-to-collision (TTC) estimates are critical to maneuver through traffic safely. Studies of relative TTC judgments showed that people perceived large, far objects as arriving sooner than small, near objects. Studies of absolute TTC judgments of laterally moving same-sized objects showed that estimates of first-arriving objects affected estimates of second-arriving objects; this effect decreased with larger temporal gaps between objects. The current study measured absolute TTC judgments of two vehicles simultaneously approaching in depth separated by 2, 5, or 7 s. Vehicles were either two motorcycles, two trucks, or a motorcycle and a truck. Results indicated that the size of a first-arriving vehicle affected TTC estimates of a second-arriving vehicle. Large temporal gaps resulted in safer judgments of the second-arriving vehicle than small temporal gaps. Thus, people should be most cautious in traffic when the first-arriving vehicle is a motorcycle and when a small gap separates two approaching vehicles.