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Amber: stop or go?

Amber alone means stop, unless you are so close that you cannot stop safely. You can work out whether you can stop: the stopping distance is the reaction distance plus the braking distance. If it is shorter than the distance to the stop line, you must stop. The higher the speed, the longer the stretch where you can neither stop nor should go.

s_{\text{stop}} = v \cdot t_r + \dfrac{v^2}{2a}

the stopping distance

Symbols

$v$	speed	m/s
$t_r$	reaction time, about 1	s
$a$	deceleration in calm braking, about 4	m/s²

Example

You are doing 50 km/h ( $v \approx 14$ m/s) and the light turns amber 40 m before the stop line.

$s = 14 \cdot 1 + \dfrac{14^2}{2 \cdot 4} = 14 + 24.5 \approx 39$ m. You can stop, so you must stop.

If the light has been green for a long time, ease off before the junction. Then you can stop calmly if it changes.

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Part of Car licence (class B): Lights, signals and traffic lights.