Over 600 taxicabs in San Francisco and Sacramento, California, were equipped with an additional brake lamp, mounted in the center of the vehicle just behind the rear window, which operated in three ways. One-third of the cabs were equipped with each of the three systems. During braking, in system- 1, the lamp flashed at 2.5 Hz;​ in system-2, it flashed at 1.5 to 7.0 Hz, discreetly proportional to the deceleration;​ and in system-3 the added lamp burned steadily. The other lamps on the vehicles operated normally. About 90% of the cabs in San Francisco and Sacramento participated in the study, and traveled almost 28 million miles.

To evaluate the effectiveness of the systems, accident reports were obtained from the drivers, detailing the precrash events. There were a total of 996 accidents, in which the cabs were rearended in 141. There were no statistically significant differences in the rate per million miles in which the cabs equipped with the three systems were rearended. No benefits could be found for the signal which provided deceleration information, although there appeared to be some benefit, though not statistically significant, for the flashing lamps as compared to the one which burned steadily during braking, in reducing those rearend crashes which occurred when the cab was stopping quickly during braking.

The study did show that the largest proportion of rearend collisions occurred when the struck vehicle was stopped, suggesting the need for a signal which indicates that a vehicle is stopped or traveling at a very low speed.