Judging speed
Assess the distance to a car together with how quickly the gap between you is closing.
Distance alone is not enough
A car can be far away and still be approaching quickly. Before pulling into its path, look at how quickly the distance between you is shrinking.
Compare what has changed
Look at the car again after a brief interval and compare it with the background and nearby traffic. This makes it easier to judge how quickly it is approaching and whether it is moving sideways.
When a car approaches from the side
Before crossing a car's path, pay attention to more than the distance to it. If it barely moves across your field of view but quickly looks larger, you may both reach the crossing point at the same time. Reassess its speed rather than counting on making it through first.
Where a second check is especially important
- pulling onto a road
- turning left
- overtaking
- moving into a lane with faster traffic
Signs of rapid closing
- a car quickly grows larger in your mirror
- the distance shrinks noticeably between two brief glances
- a car moves quickly relative to poles and nearby traffic
Speed can change
A car may accelerate or brake between your first glance and the start of your maneuver. Before crossing its path, assess how quickly it is approaching again.
Use a fixed reference point
Choose a pole, the edge of a building, or another fixed reference near the car. At your next glance, notice how far it has moved relative to that point. Movement is easier to spot this way than against an empty background.
Relative speed
Two fast-moving cars can maintain almost the same gap when their speeds are similar. If the car behind is noticeably faster than the one ahead, the distance closes quickly even when both are traveling at moderate speeds.
A single snapshot
One glance does not tell you the speed. You need to see how the distance and the car's position change between at least two brief checks.
Judging by size alone
A car's apparent size gives only a rough indication of its distance. That cue alone does not tell you how quickly it is approaching.
Follow the position of the same car
Do not compare different cars' speeds just by how large they look. Follow one car and watch how its position and apparent size change between two glances.
Look beyond the empty gap
Before moving into a gap between cars, assess both vehicles. The one ahead may slow down, while the one behind may close in quickly. An empty space at first glance is still not enough.