arcroad · night driving

Blinded by the light.

Take the driver's seat on a divided road at night. Someone is about to step out of the median gap ahead. An oncoming car keeps its high beam on. Tap "I see someone" the moment you see them.

60km/h
Oncoming car's lights
Your age
Your speed
Road

What just happened to your eyes

Scattered lightLight from a bright lamp scatters inside the eye and lays a veil over everything near it. A dark figure against a dark road loses its contrast first.
Worse with ageThe scattered veil grows with the fourth power of age: at 65 it is about twice as strong as at 25 for the same lamp.
Slow to recoverAfter the lamp passes, the eye stays dazzled for seconds. At 60 km/h every second is almost 17 metres.
Rain makes it worseA wet road mirrors each headlamp into a long streak, and the road markings vanish under it.

The rule

Rule 106 of the Central Motor Vehicles Rules requires headlamps that can be dipped so they do not dazzle the driver of an oncoming vehicle. Fines are under section 177 of the Motor Vehicles Act. A median screen or hedge on divided roads blocks the beam altogether.

How this is built, and its limits

The glare is computed, not painted. For every pixel, every frame, the veil of light scattered in the eye is worked out with the CIE General Disability Glare Equation (CIE 146:2002): Lveil/E = 10/θ³ + (5/θ² + 0.1p/θ)(1 + (A/62.5)⁴) + 0.0025p, with θ the angle in degrees between your line of sight and each lamp, A your age and p = 0.5 for brown eyes. The light reaching your eye from each lamp is its intensity divided by the distance squared, using typical values: 30,000 cd a lamp on high beam and 500 cd a lamp on dipped beam towards an oncoming driver.

"What you can see of the person" is their luminance contrast with the glare, (Lo − Lb)/(Lb + Lveil), as a share of the same contrast without it, with dark clothing at 35% of the road's brightness. Recovery after the lamp passes, the stopping distance (1.5 s to react, 7 m/s² braking) and the wet-road streaks are simplified for illustration. This is a teaching model built on published equations, not a measurement of any road, and it does not claim how many crashes glare causes.