Driver vision characteristics affecting traffic

Vision supplies about 90 percent of the information a driver uses, so its characteristics control how a road must be designed.

• Visual acuity: the ability to see detail. Static acuity is needed to read signs; dynamic acuity (seeing moving objects) is poorer and falls as speed rises. Sign letter height is chosen so that a driver of normal acuity can read the sign in the time available.
• Cone of vision: clear vision is within about 3-5 degrees; fairly clear vision within about 10-12 degrees; useful peripheral vision up to about 120 degrees (about 160-180 degrees for detecting movement). Signs must be placed within the clear cone.
• Peripheral vision: detects vehicles and pedestrians at the sides; it narrows with speed (from about 100 degrees at 40 km/h to about 40 degrees at 100 km/h) and with age and alcohol.
• Colour vision: red, green and amber must be recognised. About 8 percent of males are colour blind, so signals also use fixed positions (red on top) and signs use standard shapes.
• Glare vision and recovery: headlight glare at night reduces vision for several seconds. Recovery takes about 3 s when going from dark to light and about 6 s from light to dark, longer in old age. Hence anti-glare screens, median planting and properly aimed (dipped) headlights are used.
• Depth perception: judging distance and relative speed is vital in overtaking and gap acceptance; it is poor at night and in fog.
• Night vision and adaptation: the eye takes time to adapt to changes in light level, so lighting at tunnel entrances and underpasses must provide a gradual transition.

The design consequences are: sign size and placement, retro-reflective markings, lighting levels, sight-distance lengths, and signal lens colours and positions.