Nate Silver's FiveThirtyEight is the go-to site if you are a stats geek. Planner Gretchen Johnson and MIT PhD candidate Aaron Johnson munch the stats on bike lanes to answer the question: do bike lanes increase car traffic congestion? It's an important issue; whenever bike lanes are proposed (or a politician like Toronto's Rob Ford, who hates them, gets elected) the complaint is that obviously, if you take space away from cars it is going to slow down traffic. Except it is not so obvious.
After studying stats from Minneapolis and Brooklyn, they found that for roads which were near full capacity during peak hours, there was a noticeable increase in congestion when lanes were removed. But not all streets are at full capacity.
This is an important point: Bike lanes don’t cause a lot more congestion if you put them on the right streets. If you cut down the size of streets that are already near capacity, you’ll create severe congestion. But if you start with roads that are well under capacity, you’ll only increase the congestion a little bit. And it may not even be noticeable. Slimming down these roads that are too “fat” is known as a road diet — and yes, that is the technical term.
Now a lot of anti-bike lane types might use this as an excuse to say "go build your bike lane somewhere else, our road is at capacity," but on the very controversial Prospect Park West bike lanes, where this was an issue, they found that the installation of bike lanes didn't slow traffic much at all. They also found there were other benefits:
The number of cyclists using the road went up, and speeding cars, cyclists riding on the sidewalk and injury-causing accidents went down. The road diet isn’t just creating a space for bikers; it’s also making the street safer for other types of users.
There are also a lot of bike activists like me who would point out that who cares if drivers face a little more congestion and a minute or two longer drives, the roads are for everyone and not just cars. But that is another argument altogether.
A useful new weapon in the war on the car at FiveThirtyEight