Why straight roads appear narrow in the distance

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

06 December 2020

The farther away an object is, the smaller is appears to be. If it is bright enough to be visible, there is a distance after which it becomes a dimensionless spot. This explains why stars are points in the sky even though, in reality they are millions of kilometres in diameter.

This effect is also observed along a straight section of road. It appears to get narrower and narrower the farther out we see. The question the arises: how far away is the point where the two sides seem to meet? Well, that depends on the width of the road.

The human eye can only distinguish the width of an object if the light rays from either side make an angle greater than about 0.02 degrees. This corresponds to a thing measuring 30cm (one foot) across placed one kilometer away. If it is bright enough to be seen, it would seem like a shinny point.

Using the same ratio of 30cm wide placed 1km away, it is easy to see that, if a very bright object measuring 3m (300cm) was placed 10km, it would also become a spot in the distance.

By similar argument, an object that is 6m wide would seem like a point when viewed from 20km away. Now 6m is approximately the width of the tarmac surface on a two-lane road. Therefore, its sides would seem to meet at the far end of a straight section that is 20km long.

Do such straight roads exist in real life? Yes! The longest straight section of road in the world is to be found in Saudi Arabia. It runs for over 250km from Haradh to Al Batha.

The longest straight roads in Kenya are to be found in the flat, northern regions. The section between Marsabit and Moyale has several, but none is more than about 10km long. Therefore, we cannot see the two sides merging in the distance.

Unfortunately, even if there was a 20km straight road, you still wouldn’t see the point where the two sides merge. The reason is that, for the average adult, the horizon is just about 5km away. In other words, the 20km point if far below the horizon.

You have a better chance if you went to a railway line. The distance between the width of the standard gauge is about 1.5m, so, if you can get a 5km straight stretch, you should see the two rails merge into one at the horizon.