How wide should a bicycle lane be?
The width of a bicycle lane should be no less than 4 feet with a recommended with of 6 feet. This width does not include the road gutter section. If the road lane and the bicycle lane are placed together, the width of the combined lanes should be no greater than 14 feet. Lanes greater than that tend to imply a second travel lane to drivers. Bicycle lanes are to be located on both sides of the road on two-way streets.
Bicycle lanes less than 3 feet should not be marked as bicycle lanes. If there are rumble strips on the shoulder the width/bicycle lane is recommended to be wider to allow for a minimum of a 4 foot bicycle lane.
Lesson 19. Bicycle Lanes (pdf)
FHWA Course on Bicycle and Pedestrian Transportation