Using the high fidelity Tongji University driving simulator with 8 degrees of freedom, this study examined impacts of situational urgency on drivers’ collision avoidance behaviors. By combining different initial headways (<1.0 s, ［1.0 s, 1.5 s), ［1.5 s, 2.5 s］) and different lead vehicle deceleration rates (0.30g, 0.50g, 0.75g), rear end collision scenarios with different levels of situational urgency were established. Drivers’ perception response times (PRT), throttle release response times, throttle to brake transition times, brake delays, maximum brake pedal pressures and peak decelerations were compared across different levels of situational urgency. Results show: ①At higher situational urgency, drivers release the accelerator faster, and brake to maximum more quickly and forcefully; ②PRT was near 1.2 s when the initial headway was round 1.5 s, but PRT increased dramatically when initial headways were larger than 2.5 s, and could even reach 3 s; ③Transition time between throttle release and brake initiation is about 0.8 s and is not affected by situational urgency; ④At lower situational urgency, multi stage braking behavior leads to longer delay from brake initiation to full braking.