Two experiments are reported which examined drivers' responses in turning left across a line of traffic as presented in a closed-loop, interactive, fixed-base driving simulator. Drivers were located near an intersection and instructed to turn left across a stream of on-coming traffic when they felt that it was safe for them to do so. The on-coming stream of traffic was varied in terms of the physical parameters of approach velocity, inter-vehicle time interval, and vehicle type. Specifically, seven velocities (10-70 mph) were crossed with seven gap sizes (3-9 sec) to yield forty-nine within-subject conditions for each of four, between-subject, vehicle types; motorcycle, compact car, large car, and delivery truck. There were ten subjects per vehicle type, giving a total of forty participants in the first experiment. Results indicated differential acceptance of gap and velocity combinations depending upon the type of approaching vehicle. Collisions tended to occur along the boundaries where driver's decisions to reject or accept turns were ambivalent. They also occurred with greater frequency at higher velocity approach rates. The second experiment replicated the procedure of the first experiment except that the subjects were older drivers, uniformly over the age of 55. Results indicated a greater degree of turn conservatism for this later group. Overall, turn decisions were not dependent upon a single physical parameter such as vehicle velocity or inter-vehicle distance, although gap-size generated an arguable influence. Rather, left turn decisions appeared to result from the complex interplay of rate-of-change perceptual variables such as "time-to-arrival" and the perceived characteristics of the vehicles themselves. Implications of the results are discussed with respect to the perception of vehicles and turn safety at roadway intersections.