A few weeks ago, a sea of cyclists pedaled (slowly) through Cambridge. The city has two annual recreational rides, complete with police escorts, where hundreds of bicyclists take to the streets. I took a video of the procession through Inman Square and it got lost in my phone for a while, but recently resurfaced and made it to Youtube.
It's not that much to watch, unless you really like watching chatty cyclists at low speed, but it is illustrative how space-efficient cyclists are. The video lasts a bit shy of three minutes, and for about 2:30 of that (give or take) the swarm of cyclists passes by. I happen to know that there were about 250 (actually 258, according to the website). But let's use round numbers—even at low speed there were 100 cyclists passing by per minute.
Which is extremely efficient.
On a freeway, a lane of traffic can only carry at most 2000 vehicles per hour—beyond that the roadway devolves quickly in to gridlock (and the number of vehicles drops). Yet one lane of bicyclists can accommodate three times as many people—you'd have to have three or four people in every car on a freeway to have the same level of service. Want to fill buses? Fine, but you better be able to get a full bus through every 30 seconds to match a lane of cyclists. Rail can surpass this capacity—at rush hour the Red Line in Cambridge peaks over 10,000 people per hour, and some lines in New York go past 20,000—but you need a bit more infrastructure to run such rail routes.
Since bicycles take up far less space than vehicles, they use road space much more efficiently than vehicles. Even if a rate of 6000 per hour necessitates a police escort, it's still a testament to the efficiency of the bicycle.