Rear-end collisions have been estimated to account for 20 to 30 percent of all crashes, and about 10 percent of all fatal crashes. A thorough investigation of drivers’ collision avoidance behaviors when exposed to rear end collision risks is needed to help guide the development of effective countermeasures. Urgency or criticality of the situation affects drivers’ collision behavior, but has not been systematically investigated. A high fidelity driving simulator was used to examine the effects of differing levels of situational urgency on drivers’ collision avoidance behaviors. Drivers’ braking and steering decisions, perception response times, throttle release response times, throttle to brake transition times, brake delays, maximum brake pedal pressures and peak decelerations were recorded under lead vehicle decelerations of 0.3 g, 0.5 g, and 0.75 g and under headways of 1.5 s and 2.5 s. Results showed 1) as situational urgency increased, drivers released the accelerator and braked to maximum more quickly; 2) the transition time between initial throttle release and brake initiation was not affected by situational urgency; 3) at low situational urgency, multi-stage braking behavior led to longer delays from brake initiation to full braking. These findings show that effects of situational urgency on drivers’ response times, braking delays, and braking intensity should be considered when developing forward collision warnings systems.