Why Downhill Bike Lanes are Dangerous

Bike lanes are often promoted as “the solution” for bicycling anywhere. But there are many places where bike lanes are not appropriate and even dangerous. One such place is on a downhill where the slope is greater than about two percent.

The photo at right shows a marked bike lane on a seven percent grade. This is on Prescott Lakes Pkwy. in Prescott, AZ. About the only ameliorating factor in this bike lane is that it is so steep that few people try to ride it.

The hazard of downhill bike lanes is addressed weakly in the 2012 Guide for the Development of Bicycle Facilities, 4th Edition (AASHTO Guide):
4.6.2 Bicycle Lanes on Two-Way Streets
In most cases, bike lanes should be provided on both sides of two-way streets. …
Exceptions can be made on streets with an appreciable grade. On streets where downhill grades are long enough to result in bicycle speeds similar to typical motor vehicle speeds, then a bike lane may be provided only in the uphill direction, with shared-lane markings in the downhill direction

The Guide does not give guidance for what constitutes “an appreciable grade.” The graph below shows estimated bicycle speeds at various grades and at three different tail wind speeds for a coasting (not pedeling) cyclist. These data were calculated from this online calculator assuming a 6 ft. tall, 200 lb rider, 20 lb bike and 1000′ elevation. Note downhill slopes and tail winds are entered as negative numbers. Thus, a 6 percent slope is entered as “-6”.

At higher speeds, collisions with motor vehicles become increasingly severe. In addition, hitting a small pebble at speed can lead to a dangerous fall, possibly under the wheels of a passing vehicle. Therefore, I consider 20 mph to be a maximum “safe” speed for riding in a four or five foot wide bike lane. Without pedaling, this speed is exceeded for a three percent slope. With modest pedaling or a tail wind 20 mph can easily be exceeded on a two percent slope.

A much better treatment for a roadway such as this is to combine “May Use Full Lane” signs with shared-lane markings placed in the middle of the right traffic lane.