Increasing seat belt use: Two field experiments to test engineering-based behavioral interventions

• We completed field studies of in-vehicle systems for increasing seat belt use.
• Study 1 tested a gearshift interlock; Study 2 tested a gas pedal counterforce.
• We assessed the percentage of buckled trips and driver acceptance of each system.
• The interlock increased seat belt use, but ratings of acceptance were low.
• The counterforce increased belt use over 99%, and acceptance ratings were high.

This paper presents separate field experiments. The studies assessed engineering-based behavioral interventions to increase the frequency of drivers’ seat belt use. In Experiment 1, we instrumented a large portion of a commercial fleet with a seat belt gearshift interlock system. This system prevented drivers from engaging their transmissions unless they were buckled. The goals of Experiment 1 were to measure the change in belt use from a baseline period averaging about three months to an intervention period ranging one to three months and to assess the drivers’ acceptance of the system at the beginning, middle, and end of the intervention. The results indicated a significant increase in seat belt use from 81% to 96%, but ratings of driver acceptance were low, indicating poor acceptance. In Experiment 2, we evaluated a system that applied a counterforce that pushed against the accelerator pedal of unbelted drivers when vehicle speed exceeded 20 mile per hour (mph). Unbelted drivers could continue to drive and exceed 20mph by pressing harder than the counterforce but doing so required focused attention and physical effort. The results of Experiment 2 indicated that belt use increased from 56.2% during baseline to 99.74% during the intervention. Driver acceptance ratings were favorable. Taken together, the experiments indicate that such engineering-based behavioral interventions have considerable promise in terms of increasing seat belt use, but each faces challenges to becoming viable countermeasures. The challenges associated with the interlock appear to be attitudinal in nature on the part of the drivers, whereas those associated with the counterforce system are technical matters involving fitment of the hardware across different makes and models.