I bike on very busy multilane city streets with no issue. During rush hour, I'm usually going at or above the average speed of traffic. Have to second what Grantmeaname said . . . it's the fast roads that I avoid, busy city streets with lots of lights aren't bad at all once you get used to cycling around traffic. When the average car speed is 80 kph (50 mph) an hour that I feel much less comfortable, and will try to find other routes.