Motorist turns left in front of cyclist: 42% of bicyclists are on the sidewalk
Motorist turns left into oncoming cyclist: 15% of bicyclists are on the sidewalk
Motorist turns right into bicyclist: 31% of bicyclists are on the sidewalk
Motorist drives out of alley/driveway: 48% of bicyclists are on the sidewalk
Motorist drives through intersection: 15% of bicyclists are on the sidewalk
Bicyclist rode out intersection with signal: 24% of bicyclists are on the sidewalk

Once again, the perception is that someone is safer riding on the sidewalk than on the road - and many motorists and even law enforcement officers repeat that message. The problem is that, as the numbers above suggest, bicyclists are not safer on the sidewalk because they become almost invisible to the motorist. When a driver turns, either left or right, or into a driveway or alley, they are simply not looking for, or expecting to encounter, a bicyclist. And even if they do look and see a bicyclist they may still underestimate the speed a rider is traveling on the sidewalk - because it will likely be much faster than a pedestrian.

In addition to increasing the risk of being hit by a motorist, riding on the sidewalk also brings a rider into conflict with pedestrians. Many communities ban cycling from downtown streets so as to avoid this conflict, http://www.unl.edu/DailyNeb.arch/Fall97/Aug/082897/stories/082897/biketixs.html
and it is an issue that inflames communities and pits bicyclists against pedestrians. http://www.biketraffic.org/smithordinance.html

Furthermore, the quality of the riding surface on most sidewalks is far inferior to the parallel roadway. The vast majority of bicycle crashes that end up with the bicyclist seeking medical attention do not involve a motor vehicle and happen because a rider falls after hitting an obstacle, sliding on gravel or leaves, or loses control. Riding on the sidewalk is fraught with the kind of dangers and obstacles that may increase the chances of that happening.

Bicycle user groups and public agencies alike have produced a wealth of information and literature stressing the need for bicyclists to ride on the road and not the sidewalk. Examples include:

Pennsylvania Department of Transportation: http://safety.fhwa.dot.gov/fourthlevel/pdf/pamanual.pdf
Bicycle advocates in Northeastern Ohio: http://www.crankmail.com/bike-psa.html
The city of Saskatoon, Saskatchewan: http://www.city.saskatoon.sk.ca/org/municipal_engineering/cycling/sidewalks.asp
Boston area bicyclists: http://www.massbikeboston.org/tips/traffic.htm

Another answer to the problem of riding on sidewalks is to improve conditions on the roadway so that bicyclists feel safer and more comfortable riding in traffic. The range of possible improvements includes the striping of bike lanes, road surface improvements, and traffic calming to reduce vehicle speeds and volumes. (Go to Chapters 19 and 20 of FHWAs Course on Bicycle and Pedestrian Transportation)