1. A system for avoiding collision between a first vehicle and a second vehicle each capable of moving along the same general path, said system comprising at least one means mounted on said first vehicle for emitting optical pulse signals; detection means mounted on said second vehicle including optical scanning means, the scanning operation thereof defining a substantially triangular field of scan, said scanning means being responsive during its scanning operation to the optical pulse signals emitted by said at least one emitting means for providing output scanned information; means responsive to said output scanned information for determining the range between said first and second vehicles, means responsive to changes in said range determination as a function of time for determining the closing speed between said vehicles, and alarm means responsive at least to said closing speed determination for permitting evasive action to be taken by said second vehicle when said closing speed exceeds a selected value to avoid collision with said first vehicle.