Two photos taken from Lake Shore Drive at Belmont, looking south, 1951, Chicago.
Notice the three wide parallel lines that run along the drive. These were called “fins” and could be hydraulically raised or lowered depending on the time of day to regulate traffic patterns.
The first photo was taken between 6:30-9:30 am. One fin would be raised, creating 2 lanes of north bound traffic and 6 (!) lanes of south bound traffic.
The second photo was taken between 4 and 7 pm, showing 6 lanes designated as north bound (with three “express” lanes in the center) and 2 lanes as south bound.
During non-rush hour traffic, only one fin would be raised in the center, creating 4 north bound and 4 south bound lanes - the permanent configuration we have today.
I imagine this had the potential of creating chaos as the fins appeared or disappeared, but there are many mornings during my standstill commute downtown that I wish this was still in practice!