Science Puzzle
Why Are Tunnel Lights Brightest at the Entrance?
Drive into a road tunnel on a sunny day and the lights near the entrance are packed close together and very bright. Further in, they're spaced out and much dimmer. At night, the entrance lights are turned right down.
Why do tunnels put their brightest lights at the entrance?
The Answer
Drivers arrive with eyes adjusted to bright daylight, and eyes take time to adjust to the dark. The bright entrance gives them that time.
A sunny road can be hundreds or even thousands of times brighter than the inside of a tunnel. Your eyes cope with that range by adjusting: the pupils widen and the cells in the retina become more sensitive. That takes time. If a tunnel were dim right from its mouth, the entrance would look like a black hole to a driver arriving from the sunshine, and anything inside, such as a broken-down car, would be invisible until it was too late.
So engineers step the light down. The first stretch, called the threshold zone, is lit almost like daylight. The light then falls gradually, at about the rate a driver's eyes can follow, until it reaches the steady level used for the rest of the tunnel. Sensors outside many tunnels measure the daylight and dim the entrance on dull days. At night the outside is dark, so the entrance needs hardly any extra light at all.
The exit is less of a problem. Eyes adjust to brightness within a second or two but take far longer to adjust to darkness, which is why walking into a dark cinema on a sunny afternoon leaves you briefly blind.
The principle: Light adaptation. The eye adjusts its sensitivity to the brightness around it, quickly when things get brighter and slowly when they get darker, so tunnels lower their lighting in steps.