From John S. Allen, LAB Regional Director, New York/New England:
"The evidence that bicycling on sidewalks and similar facilities is more hazardous than bicycling on streets is overwhelming."
From "Adult Bicyclists in the U.S." by Dr. William Moritz:
Relative danger index 24.8 times as high for sidewalk riding as for major street without bicycle facilities. (ADULT BICYCLISTS IN THE UNITED STATES - CHARACTERISTICS AND RIDING EXPERIENCE IN 1996, William E. Moritz, Ph.D., Professor (Emeritus) Human Powered Transportation, University of Washington, Seattle WA)
According to Dr. Eero Pasanen, Helsinki City Planning Department, Traffic Planning Division, HELSINKI
"A recent study in Helsinki showed that it is safer to cycle on streets amongst cars than on our two-way cycle paths along streets. It is hard to imagine that our present two-way cycling network could be rebuilt. But in those countries and cities which are just beginning to build their cycling facilities, two-way cycle paths should be avoided in urban street networks.
Even in more advanced cycling countries like Denmark and in the Netherlands, with a lot of cyclists and with their one-way lanes and paths, cycling is still much more dangerous than car driving or public transport. "
Alan Wachtel and Diana Lewiston published in the ITE Journal, Sept/Oct 1994 (from the Institute of Transportation Engineers):
"The average cyclist in this study incurs a risk on the sidewalk 1.8 times as great as on the roadway. The risk on the sidewalk is higher than on the roadway for both age groups, for both sexes, and for wrong-way travel. The greatest risk found in this study is 5.3 times the average risk for bicyclists over 18 traveling against traffic on the sidewalk."
"Wrong-way sidewalk travel is 4.5 times as dangerous as right-way sidewalk travel. Moreover, sidewalk bicycling promotes wrong-way travel: 315 of 971 sidewalk bicyclists (32 percent) rode against the direction of traffic, compared to only 108 of 2,005 roadway bicyclists (5 percent)."
"Even right-way sidewalk bicyclists can cross driveways and enter intersections at high speed, and they may enter from an unexpected position and direction for instance, on the right side of overtaking right-turning traffic. Sidewalk bicyclists are also more likely to be obscured at intersections by parked cars, buildings, fences, and shrubbery; their stopping distance is much greater than a pedestrian's, and they have less maneuverability."
There are of course many more supporting expert opinions which are easily found. These only scratch the surface, but should be more than enough to convince a novice.