There are two optical phenomena that need to be recognized in street light installations.
The loss of night vision because of the accommodation reflex of drivers' eyes is the greatest danger. As drivers emerge from an unlighted area into a pool of light from a street light their pupils quickly constrict to adjust to the brighter light, but as they leave the pool of light the dilation of their pupils to adjust to the dimmer light is much slower, so they are driving with impaired vision. As a person gets older the eye's recovery speed gets slower, so driving time and distance under impaired vision increases.
Oncoming headlights are more visible against a black background than a grey one. The contrast creates greater awareness of the oncoming vehicle.
Stray voltage is also a concern in many cities. Stray voltage can accidentally electrify light poles and has the potential to injure or kill anyone who comes into contact with the pole. [29] Some cities have employed the Electrified Cover Safeguard(TM) technology which sounds an alarm and flashes a light, to warn the public, when a pole becomes dangerously electrified.
There are also physical dangers. Street light stanchions (poles) pose a collision risk to motorists. This can be reduced by designing them to break away when hit (frangible or collapsible supports), protecting them by guardrails, or marking the lower portions to increase their visibility. High winds or accumulated metal fatigue also occasionally topple street lights.
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