So you're out on the lake in your dinghy, or lost on the Bonneville Salt Flats in your go cart, and you spy a dot on the horizon. You know it's not a piece of dirt on your spectacles; you're getting an ominous feeling that it's a cigarette boat or a rocket-propelled car that could slice your pedestrian craft in half.

Should you be worried? Assuming both of you are traveling at constant velocity, here's how to tell. Take a bearing on the approaching vehicle; wait a bit (how long depends on how close it is), and take another bearing. If the two bearings are the same, you've got a problem.

An easy way to do this, without resorting to a compass or other technological crutch, is to simply look at it for a while and see if you have to move your head (or eyes) to keep it in the same place in your field of vision.

The rule of thumb, therefore, is: Constant bearing implies collision

There are two self-evident exceptions to the rule:

  1. If you're motionless relative to the other (i.e., your velocities are the same, or you're both stationary (a special case of a special case))
  2. If the paths are colinear and the separation between the vehicles is increasing