Since driving is in decline, perhaps removing car lanes for bike lanes could free up room on space-constrained urban streets. How much more traffic could these streets serve? In spite of grumblings to the contrary, bike lanes are much more efficient at moving vehicles than traffic sewers are. (Remember that bikes are vehicles, too.) Consider first that a single road lane can typically move at most 1,000-2,000 cars per hour — the upper end for expressways and the lower end for arterials. (Local streets move fewer than 1,000.)
Removing one car lane can create enough space for two buffered bike lanes, or for one bidirectional cycletrack. Each of those lanes, in turn, could easily move almost twice as many vehicles as each car lane:
[T]he saturation flow for a single 1-m (3.3 ft) to 1.2-m (4-ft) bicycle lane appears to be between 1,500 and 5,000 bicycles/hr with a majority of the observations falling between 2,000 and 3,500 bicycles/hr. (D. P. Allen, N. Rouphail, J. Hummer, J. Milazzo, TRR 1636)
In other words, converting one lane to a cycletrack can quadruple the capacity of that lane of traffic. It’s like adding three lanes of traffic, just with some paint. The inverse of that statement: even if the lane “looks” 75% less busy than the old lane of traffic, it’s still moving about as many vehicles and just about as many people (since average car occupancy is ~1.25).
west north 
Those are very narrow bike lanes!
The narrowest lane in Chicago is 5 feet wide and several are 6 feet wide. Bike lanes in the Netherlands are often 2 meters wide.
I disagree that one regular lane (they’re not always car lanes because people can ride bikes in them) can fit two buffered bike lanes, at least with bike lanes that have the minimum Chicago width of 5 feet.
Depends on the lanes, of course. Many arterials have lanes 10-12′ wide, and some go as wide as 14′ (particularly where “wide outside lanes” are used as bicycle accommodations). Two 5′ bike lanes fit nicely into one 10′ lane. Buffered cycletrack lanes can also be narrower than bike lanes, and one-car-lane-to-cycletrack conversions have certainly been done.
I’ll have to look at my Montreal et al guides just to be sure, but Chicago’s bike lanes are indeed wider than they are elsewhere — largely since they’re often placed next to the door zone. Not all cities, particularly those laid out before the 19th century, have nearly as much space as Chicago.