A study was conducted to investigate drivers’ reactions to tread separation scenarios using the National Advanced Driving Simulator (NADS). The objectives were to evaluate the effects of vehicle understeer gradient, prior knowledge of an impending tire failure, instructions on how to respond to a tire failure, driver age, and failed tire location on drivers’ responses and the likelihood of control loss following simulated tread separation on one of the rear tires of a simulated SUV traveling at high speed.

One hundred and eight (108) subjects experienced two tire failures while driving on a straight divided highway at approximately 75 mph with light surrounding traffic. Subjects were divided equally into three age groups (18-25, 35-45, 55-65) and gender was balanced. Drivers were assigned to one of three understeer conditions. Understeer conditions were referred to as Vehicle 1 (understeer gradient of approximately 4.7 deg/g ), Vehicle 2 (3.4 deg/g), and Vehicle 3 (2.4 deg/g). Following left rear tire detread, the understeer gradients resulting from a right turn changed to 1.10, 0.09, and –1.17 deg/g, respectively. The first tire failure was unexpected. The second tire failure was expected, with half of the subjects being given specific instructions on how to respond to a tire failure and the other half were told only that one or more tire failures would likely occur.

Decreasing vehicle understeer was strongly associated with the likelihood of control loss following both the unexpected and expected tire failures. Knowledge of the imminent tread separation reduced the overall probability of control loss from 55% to 20% and had a significant effect on how quickly drivers responded as well as on the nature of their initial responses (i.e., steering or braking). Driver age was marginally associated with increased likelihood of vehicle control loss, but only on unexpected trials. Vehicle speed at the time of first steering input also contributed to the probability of control loss. Neither the location of the tire that failed (left rear vs. right rear) nor the specific instructions about how best to respond to the tread separation influenced the probability of control loss. Differences associated with vehicle understeer conditions observed in the present study were large and consistent, independent of driver expectations and across driver age groups. It is thus fair to conclude that in the event of a complete rear-tire detread, the increased difficulty in vehicle handling and the associated increased likelihood of loss of vehicle control with decreasing vehicle understeer generalize to real-world driving.

A complete description of the methods used and results of this research are contained in the NHTSA report, "Investigation of Driver Reactions to Tread Separation Scenarios in National Advanced Driving Simulator (NADS)" DOT HS 809 523" released in January, 2003.