Driver-and-Vehiele Response in Freeway Deceleration Waves

By a continuous circling technique, 35 mm air photo records were made during 10 peak traffic hours on two sections of an urban freeway, 5 on a four-lane, and 5 on a three-lane out bound section. Speeds, time headways, distance headways, and response times were analyzed from samples selected from the 1,000 ft of projected films to include deceleration waves. Each sample included from 40 to 150 pictures spaced 1 sec apart yielding 20 to 30 individual measurements for a total of 3,000 to 4,500 in each sample. All measurements were corrected by electronic computer for continuously changing camera angle and slant range (including altitude). Of the samples analyzed, eight from the four-lane section and two from the three-lane section showed typical deceleration waves. Three on the latter showed dissipation of “slowdowns” with increasing speeds and decreasing headways. The four-lane section included speed control signals but the three-lane section did not. On the former, incoming ramp vehicles, visible slowing ahead and signals apparently caused drivers to operate in an uncertainty mode more than on the latter. Weaving vehicles were more numerous on the latter. Longer average time headways were found after deceleration waves. Longer headways related to driver uncertainty are consistent with previous reports and are of importance for traffic flow theory and for freeway design, control, and operation.