But Mom said to ride on the sidewalk!

When you were a small child, you rode at a walking pace, and you didn't know how to look out for yourself. Riding on the sidewalk may have been the best choice for you then -- though it never actually was very safe.

Also, you only went swimming at the shallow end of the pool when you were a small child. But most teenagers and adults can swim with confidence at the deep end. It's the same thing with bicycling. Grownups can look out for themselves, ride faster, and are ready to handle riding in the street.

You wouldn't want to try diving into the shallow end of the swimming pool. Similarly, though it may not seem to make sense at first, adult bicyclists have a far higher risk of crashes on sidepaths than on streets. [Detailed explanations, and survey of research]. MIT and the City of Cambridge have flown in the face of the research record and design guidelines not only in constructing the sidepaths, but also through a number of unsupportable claims and broken promises.

So the best advice is to ride in the street, according to the rules of the road -- a detailed tutorial is available on the Web and in print: www.bikexprt.com/streetsmarts

Also, there is no law requiring you to ride on a sidepath. But if you are going to ride on one, take this advice:

Ride slowly so collisions with pedestrians aren't too serious. The car-bike crash rate for through travel on a sidepath is about 3 times at high at intersections and driveways when traveling with traffic, and 12 times as high when traveling against traffic, so be careful! Be VERY careful of blocked sight lines: doorways hidden behind groups of pedestrians, portals where a car might be coming out of a driveway where you can't see it, etc. etc.

Stop at every driveway and look over your shoulder, and walk across, because there is no other way you can be sure to avoid the motorists turning across your path. They also will have a hard time seeing you, because of the row of trees and the parked cars between you and them. Oh, and by the way, when you go ride in the rest of Cambridge, it's completely different: there you ride in the street and follow the regular traffic laws.

Remember that elsewhere in Cambridge and throughout the region, it's against the law to ride on the sidewalks in some places, and discouraged in most others.

It is a common belief  that the main danger to bicyclists is from overtaking motor vehicles, and that separating bicyclists from the flow of motor traffic will greatly increase safety. In fact, most injury-producing bicycle crashes do not involve a motor vehicle at all, and most car-bike collisions result from crossing and turning movements rather than overtaking. Sidepaths increase the collision rate by placing bicyclists and motorists out of view of each other and on conflicting paths until it is too late to avoid a collision. This is especially so in urban areas where traffic is slow and there is lots of crossing and turning.

Sidepaths also have serious maintenance problems, as well as conflicts between bicyclists and with pedestrians.

So please take my advice.