Idaho Roadway Design Manual (2013) [PDF]
pp. 41-43
Shared Lane
On a shared facility, bicyclists and motorists share the same travel lanes. Shared facilities are common on city street systems and roads with limited right-of-way. Shared lanes can be considered an acceptable solution when there is inadequate width to designate bike lanes or provide shoulder bikeways.
A lane with 14 feet of usable width is desired in an urban setting, which allows a motor vehicle and a bicycle to operate side by side. Usable width normally would be from edge stripe to lane stripe or from the longitudinal joint of the gutter plan to lane stripe (the gutter pan should not be included as usable width). Widths greater than 14 feet may encourage the undesirable operation of two motor vehicles in one lane. In this situation, consideration should be given to striping a bicycle lane or shoulder bikeway. ** Shoulder Bikeway Smooth, paved roadway shoulders provide a suitable area for bicycling, conflicting little with faster-moving motor vehicle traffic. Rural bicycle travel on the state highway system is accommodated on shoulders. Optimum shoulder width for bicyclist allows for a minimum 4 ft of clear path that is void of rumble strips, gutter pans, grates or other surface discontinuities. Shoulder areas against an ordinary curb face should have a 5 foot minimum width or 4 feet from the longitudinal joint between a monolithic curb and gutter and the travel way. Shoulder widths of 5 feet are recommended from the face of a guardrail or other roadside barriers. Adding or improving shoulders can often be the best way to safely accommodate bicyclists in rural areas, and they are also a benefit to motor vehicle traffic. In severely restricted areas, even minimal width shoulders, 2-3 foot, is an improvement over no shoulder at all.
Bicycle Lane
A bicycle lane is a portion of the roadway that is designated for preferential use by bicyclists. Bicycle lanes are very common in urban areas and must always be well marked and signed to call attention to their preferential use by bicyclists (refer to Manual on Uniform Traffic Control Device & AASHTO Guide for the Development of Bicycle Facilities).
Bicycle lanes are established on urban arterial and major collector streets. The minimum width for a bike lane is 4 feet, or 5 feet from the face of a curb or guardrail. A clear riding zone of 4 feet should be present if there is a longitudinal joint between the pavement and the monolithic curb and gutter section. Bicycle lanes in excess of 6 feet in width are undesirable as they may be mistaken for a motor vehicle lane or parking area. Refer to the Traffic manual or the Manual on Uniform Traffic Control Devices & AASHTO Guide for the Development of Bicycle Facilities for detailed specifications for pavement striping, stenciling, and signing of bicycle lanes.
*Min: 5 ft. against curb or guardrail, 5 ft. lane next to parking If parallel parking is permitted, the bike lane must be placed between the parking area and the travel lane and have a minimum width of 5 feet. Bicycle lanes must always be one-way facilities and carry bicycle traffic in the same direction as adjacent motor vehicle traffic. On one-way streets the bicycle lane should be on the right side of the roadway, except in areas where a bike lane on the left will decrease the number of conflicts (e.g., those caused by heavy bus traffic or dual right-turn lanes, etc.).