Why is it easier to cross the road in daylight than at night?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

01 March 2009

Martin Kamande has observed that it is easier to cross the road during the day that it is at night! Now you might be tempted to retort “everybody knows that!” but not after read his reasoning.

He says: “By looking at the body of the vehicle during the day and [listening to the] sound of the engine, I am able to judge the speed. That is, [change in] perspective and Doppler Effect come to the rescue …. In the evening I have to rely on the vehicles headlights [and] … most of the time I end up misjudging the speed of the vehicle.

“I reasoned that since light doesn't exhibit Doppler Effect… the headlights will not appear to brighten as the vehicle approaches so I can’t judge the speed.   Now, could this observation proof that the speed of light is always a constant?”

If the reader finds that explanation confusing, it is because Martin has mixed up the basic principles. First of all, Doppler Effect is the observed change in the tone of the sound of a vehicle as it passes by.

If you are standing by the roadside, the sound of an approaching vehicle will be a higher pitch than normal but after the car passes by and drives away the noise suddenly changes to a lower tone.

I don’t have enough space in these columns to give the complete scientific explanation for the effect, but, suffice it to state that it is observed in all kinds of waves – including light waves. Thus the first part of Martin’s reasoning is incorrect.

Light exhibits Doppler Effect and it is one of the principles used by astronomers in measuring the speeds and distances of far away galaxies. There is a direct relationship between the velocity of the source of the waves and the change in tone – in the case of light waves, change in colour.

In addition, Martin’s observation is not proof that the speed of light is always a constant. The reason he is able to judge the speed of an approaching car during the day is that there is enough sunlight to illuminate the car and its surroundings.

Therefore, as he watches the car, his eyes also note its position relative to the trees, bushes and other objects on the roadside. All that information goes to his central processing unit (the brain!) where it is compared to other historical data collected from similar situations. The result is a fairly accurate assessment of whether he can cross the road before the vehicle reaches his crossing point.

During the night, there isn’t enough light to illuminate the road environment. So Martin’s brain gets incomplete data and miscalculates the desired result. The good news is that some one at the City Council has noted martin’s problem. The new street lights under installation along Uhuru highway have two bulbs – one is directed over the road and the other is illuminates the sidewalk.