Time to collision (TTC)
An estimate of how many seconds remain until two moving objects would meet if they held course — the basis for predictive avoidance.
Reactive avoidance, which looks only at the distance right now, makes agents collide or jerk away at the last moment. TTC looks ahead: given two objects' positions, velocities and radii, you solve for when the gap between them would shrink to the sum of their radii. If that time is infinite the paths never cross; if it is small, a collision is imminent.
Agents weight their avoidance by TTC — a threat two seconds away dominates the steering, one twenty seconds away is ignored. That keeps motion smooth and anticipatory rather than panicked. TTC underlies the velocity-obstacle methods, including RVO, on which modern crowd simulations are built.