h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Well, some mice, anyway.

The red light is always on top for two reasons :

1) It is a standard so that people who are red/green colourblind can distinguish the difference between the two by position.

2) Because red is the most important state of the traffic lights (since if both directions are on red there are no crashes, but if both are on green there could be lots of crashes), it goes at the top so that it is less likely to be obstructed by tall(er) vehicles/objects.

Interesting thoughts

As it happens I work on the railway in the UK.

With railway signals the red light is positioned closest to the drivers line of sight, and is therefore normally the bottom aspect, If a signal has to be mounted low down then the red becomes the top aspect.

The other reason given for the red aspect being the bottom one is that in remote areas snow could build up on the lens hoods obscuring higher aspects.

I think you are right about road traffic lights having the red at the top to aid colour blind drivers but I think this is a convention which started by chance with the first traffic lights and has been retained.

I can't get my head round your point (2)surely if a tall vehicle is obstructing the lights it is also blocking your way.