Ultimately, the environmental effect of bike lanes will take more study, as pointed out elsewhere. But it does appear that they make traffic a little safer. A decade ago, the Chicago Department of Transportation surveyed bicycle program coordinators in multiple other cities, and not only found that dedicated lanes on 44' streets were possible, but that other cities, including Toronto, Portland (Ore.), Philadelphia, Cambridge (Mass.), and Madison (Wis.), reported that bike lanes or guide lines appeared to have a calming effect on automobile speeds. See http://www.bikeplan.com/narrow.htm For the Cambridge study results, see http://www.cambridgema.gov/~CDD/et/bike/bike_safety.html Chicago then began a trial program of putting in bike lanes in high-volume streets as narrow as 44'. When the City found that the lanes reduced crash severity without degrading auto traffic, they expanded the program, which is one of the reason you see so many bicyclists in Chicago. http://www.activelivingresources.org/assets/chicagosbikelanedesignguide.pdf A recent study at the University of Texas, founded by the Texas Department of Transportation, found that bike lanes increase safety for both cyclists and auto drivers by decreasing the tendency of drivers to "over-correct" and swerve when passing a bicycle. Previous studies have also shown that bicyclists are more likely to stop at intersections and obey other traffic rules when roadway markings accommodate them. http://www.utexas.edu/news/2006/09/18/engineering/ Bikes and cars are different, and ultimately may require different rules, maybe even different roadways in some areas. But the evidence suggests that giving bikes a share of the road benefits everyone in the long run.