Radar-Measured Reaction Times of Unalerted Drivers to Brake Signals

This article concerns the responses of drivers to brake signals in actual traffic. The data were collected in a project investigating the potential benefits of high-mounted brake lights. The responses were obtained from unalerted drivers who at the time of the trial were following behind a test car and in front of a monitoring car. The test car (a compact or a full-size) was equipped on 69% of the trials with one or two supplemental brake lights, mounted just below the rear window or at roof level. A Doppler radar antenna was mounted at the center of the front bumper of the monitoring car. The radar monitored changes in the speed of the subject's vehicle and generated a trace on one track of an event-recorder. A telemetered input from the test car produced a deflection on the second track of the tape, corresponding to the start and end of the brake signal. The trials were run on two different routes, urban and suburban (speed: 32 to 40 km/hr and 56 to 72 km/hr, respectively; inter-car spacing: 1 to 2 car lengths and 3 to 5 car lengths, respectively). Only trials meeting all of the following criteria were included in the analysis: (a) The subject's vehicle did not appear to have braked or decelerated in response to vehicles ahead of or adjacent to the test car, to changes in roadway configuration or a traffic control device, or in preparation for turning off the roadway. (b) The roadway was generally flat. (c) The subject's vehicle was a car, van, or pickup truck. A total of 1,644 data points were collected. On 72% of the trials there was a discernible speed change within the 3 sec. following the signal onset, with a mean delay of 1.21 sec. and a standard deviation of .63 sec. By including no-responses and responses longer than 3 sec, the distribution of the speed-change delays under the present conditions can be characterized by a 72 percentile value of 3 sec.