Resources: (Information from 1992)
Manual on Uniform Traffic Control Devices, 1988, U.S. D.O.T., approx. $22.00 from the Superintendent of Documents, U.S. Government Printing Office, Washington DC 20402 [Replaced by 2003 Version]
Bikeway Planning and Design Standards, 1987, free from Rick Blunden, CalTrans, 1120 N St, Sacramento CA 95814
Policy & Procedure for Bicycle Projects, 1988, free from the Ohio Department of Transportation Office of Bicycle Transportation, Rm. 418, 25 S. Front St., Columbus OH 43215
Left: This approach to bike path/roadway intersections makes it less inviting for motorists to drive on to the path while keeping bicycle obstructions to a minimum.
Below: California's approach to striping around a barrier post in the middle of a path.
Bottom: A stationary post in the middle of a path.
Published August 1992
Author: John Williams
Originally published in Bicycle Forum (Issue 30, August 1992), now NCBW Forum.
The best barrier is routine enforcement. While some communities install bollards, these are easy for bicyclists to miss (visually) and hit (physically). Tom Walsh, Traffic Engineer for Madison, Wisconsin, only uses bollards when there's a proven need.
Otherwise, he doesn't use anything more than regulatory signs. If you do use bollards, make sure they're highly visible (reflective tape, overhead lighting, etc). If you need to get through with emergency vehicles, consider a hinged design for the post. Use the Caltrans approach to striping around them (see diagrams below right).
Diane Bishop reports that Eugene, Oregon, uses a different approach: they split the path entrance into two 5-foot one-way paths. In between the paths, they plant low bushes to discourage motorists; fire trucks and such can drive over these bushes in an emergency. We've reproduced a diagram based in part on one found in the Ohio Department of Transportation's design guide to give you the idea.
Enforcement is the only way we've seen. If you make a barrier that will keep out a motorcycle, it'll also be a real nuisance for bicyclists and (on multiuse paths) wheelchair users.