nent bridge construction and for temporary and permanent lane separation when there is not enough distance between opposing lanes.
Early interstate highway designs left unprotected those bridge supports and columns
located at some distance from the pavement. The columns were immoveable when struck.
Occasionally, people used them to commit suicide by driving into them at high speeds.
Today, energy-absorbing barriers containing sand or water protect some of them;
guardrails and the standard concrete barriers protect others. Figure 14-6 illustrates pro-
tection of bridge piers.
Environments
Traffic safety environments are comprised of such factors as time of day, weather conditions, pavement conditions, and other factors.
Accidents occur more frequently at night. Not only is visibility a problem, but the
proportion of drivers who have been drinking is higher at night.
Other factors contribute to the increase in nighttime accidents. Lighting of highways
and intersections in particular reduce accident frequency. Light from headlights dimin-
ishes with the square of the distance from the vehicle. Although objects are visible at some
distance, the driver has a limited amount of time to react and stop the vehicle. The dis-
tance required for perception, reaction, and stopping must be less than the effective