Convex mirrors belong on all new cars

Convex mirrors are a perenial favourite topic from people writing to us. People either love them or hate them and want to tell us how they feel.

Convex mirrors are more expensive to make than flat ones so there must be a good reason for fitting them

First things first. Car makers are amongst the lousiest business on earth, they cost everything to the fraction of a cent. Convex mirrors (mirrors with a curved glass) are more expensive to make than flat ones so there must be a good reason for fitting them. Well it so happens there is. The reason comes from studies in US where the authorities faced the same dilemma of acceptance as Australia and from Europe where where convex glass is used on the majority of vehicles. The research compared the benefits of convex, aspherical (mirrors with both curved and flatter glass) and planar (flat glass) mirrors. The results were quite surprising and very consistent.

Objects in the mirror are closer than they appear” may not actually be true

The long and the short of it is that distance perception in a mirror is actually very poor, in any sort of mirror. The good news is, people almost invariably believe things in a mirror are closer than they really are. In other words the warning “Objects in the mirror are closer than they appear” may not actually be true since we’re dealing to an abstract perception not a real measurement.

Examples of relfections to see in different shaped wing mirrors

A more accurate statement would be “Objects in the mirror are larger than they appear”.

In pure optical physics sense, convex mirrors reduce the image size. Based on this, the naive assumption from legislators of yesteryear was that image size equates directly to distance perception. In fact it seems the vision perception part of our brains is far more sophisticated than that. We are able to use reference cues other than simply the size of the object to give it context. For this reason I always recommend when setting you mirrors to leave a portion of your car visible as a fixed reference.

studies show there is no significant difference in any mirror type when judging a safe gap to merge into traffic

In terms of perceiving a safe gap when merging into adjacent traffic the studies actually show that there is no significant difference in any mirror type. In fact the only trend shown by the study was related to the age of the participant. Older drivers it seems are much more likely to allow a larger gap. We learn from this that drivers do not typically rely on the image in exterior mirrors to gauge distance.

The conclusion is that drivers do not typically rely on the image in exterior mirrors to gauge distance.

convex mirrors increase your field of view by up to 50% higher over a flat plane mirror

However, convex mirrors (and asphericals) have a huge advantage in field of view, up to 50% higher than a flat plane mirror. This greatly reduces blind spots which may account for the fact that cars equipped with convex or aspherical mirror types were found to be under-represented in crash statistics where field of view was a factor. The study reviewing this discovered such accidents were 20% more likely in a car with flat plane mirrors.

Do you disagree? See ‘Convex mirrors don’t belong on new cars‘ for a different point of view.

Images iStock.com /tzahiV

Written by Nicholas Platt, Senior Vehicle Engineer
July 14, 2016