Position of Curb. Curbs are normally used at the edge of pavement on urban streets
where the design speed is 40 mi/h (64 km/h) or less. Curbs at the edge of pavement
have an effect on the lateral placement of moving vehicles. Drivers tend to shy away from
them. Therefore, all curbs should be offset at least 1 ft (0.3 m) and preferably 2 ft (0.6 m)
from the edge of the traffic lane. Where curb and gutter are used, the standard gutter
width is 2 ft (0.6 m).
On roads where the design speed exceeds 40 mi/h (64 km/h), curbs should be used
only in special cases. Special cases may include, but are not limited to, the use of curb
to control surface drainage or to reduce right-of-way requirements in restricted areas.