杏吧原创

FAST driving in foggy weather shouldn鈥檛 be blamed entirely on motorists鈥�
recklessness. 杏吧原创s in Cardiff suggest that a trick of
perception鈥攚hich makes people driving in fog feel they are going more
slowly than they really are鈥攑lays a role.

Robert Snowden and his colleagues at the University of Wales at Cardiff
showed volunteers videos filmed through a car windscreen. Each volunteer was
shown one scene taken on a clear day and another filmed during foggy weather,
and was asked to judge when the scenes were flitting by at equal speeds. The
subjects consistently saw the foggy scenes as passing by more slowly, Snowden
reports in this week鈥檚 Nature(vol 392, p 454).

The researchers then trained volunteers to drive at specific target speeds on
a simulator. 鈥淭o begin with people are absolutely hopeless at this,鈥� says
Snowden. But they were soon able to accurately gauge their speed when in a clear
weather scene.

Not so in fog, however. 鈥淲hen we asked them to drive at 50 miles per hour,鈥�
says Snowden, 鈥渢hey drove at 65 or 70. Some were 50 per cent over.鈥� The same was
true at higher speeds: asked to drive at 70 miles per hour, motorists in the
foggy simulation accelerated to 90 miles per hour.

Snowden says that his volunteers were convinced that their speed was not
excessive. 鈥淧eople know they should slow down, and they think they have,鈥� he
says.

Faulty speed perception in fog may be due to reduced contrast, Snowden
believes. Other studies have shown that a simple pattern of black and white
stripes moving at a constant speed appears to move slower when contrast is
reduced. Fog likewise reduces contrast. 鈥淧erhaps the cells responsible for
encoding speed confuse changes in contrast with changes in speed,鈥� says Snowden.