By a continuous circling technique, 35 mm air photo records were made during 10 peak traffic hours on two sections of an urban freeway, five on a four-lane, and five on a three-lane out-bound section. (all measurements were corrected for continuously changing camera angle and slant range, including altitude.) Speeds, time headways, distance headways, and response times were analyzed to include deceleration waves. 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. /author/.
Abstract