Orange lights denote the “threshold zone”, meant to help drivers’ eyes adjust to the lower light levels inside the tunnel.
When you’re driving through tunnels on the Central Expressway or the Kallang-Paya Lebar Expressway, have you noticed the lighting in them?
When you first enter, the light is a deep orange, which gradually turns into white light.
Then, as you prepare to exit the tunnel, the light becomes purely white.
Have you ever wondered why?
Why are lights in the tunnel differently coloured?
To understand why some lights are orange, we first must understand how the human eye works.
Specifically, we must understand how transitioning from bright to dim light affects visual clarity.
Our eyes take time to adjust to darkness – just try walking from a very bright hall to a dark room when you’re at home.
Your eyes struggle but eventually focus in the dim light.
Now, transpose that situation to one where you’re travelling at 90km/h, and it becomes much more immediate to address that situation.
Tunnel lighting must allow drivers to enter, transit the tunnel and exit safely and comfortably.
Orange lights
Orange lights are usually found at the beginning of the tunnel, called the “threshold zone”.
The orange lights have two purposes: to let drivers see and avoid obstacles, and once in the tunnel, to adjust to the lower light levels.
Orange and white lights
This is called the “transition zone”, and as the name suggests, it’s when the driver’s eyes are beginning to adjust to the lower light levels inside.
The reduction in the light must be gradual and not exceed the eye’s capacity to adapt to the environment.
White lights
White lights denote the “interior zone”, the longest stretch of road.
This is called the “interior zone”, and is the area between the transition and exit zones.
This is the longest stretch, and the lighting must be sufficient for the speed that cars are allowed to travel inside the tunnel.
Exit zone
According to studies, the eye adapts instantly when transitioning from low to bright light, and so doesn’t need special adaptative processes.
So the next time you enter a tunnel and wonder why there are orange lights, it’s to help adjust your eyes so that you can avoid obstacles easily.
