Why don't they stagger the shifts so that not all taxicabs are off-duty between 4 and 5 P.M.? That is one of the most common questions I get as a hack. My guess is perhaps only slightly better than yours. I've heard it has to do with ensuring that no shift gets the short end of the stick. Each of the two 12 hour shifts needs one uninterrupted chunk of rush hour in order to make do. It seems that in order to make things even, whoever created the universal shift change decided to interrupt the afternoon rush of just about anyone and everyone. If I were in charge, I'd stagger the shifts and create different leasing prices to adjust to that shift's attractiveness.
I personally maintain a schedule that is staggered from the rest of my fellow drivers. I often go into the garage by 2 A.M., sometimes as early as 1:00. The dispatcher hands me whichever cab happens to be available at that time, and I return it exactly 12 hours later. That means I'm back in the garage by 1 or 2 in the afternoon, hence making the taxi available to any night driver that wishes to begin that early, which in turn lessens the distress that the city feels at the height or the homebound rush.
I don't get any discount on my daily lease for getting my butt out of bed so early or for taking that fiscal risk of being out on the streets at the quietest hours, gambling with my gas tank and cruising time, trying to eek out a miracle or three. I don't mind though. I love the rush I get from this challenge. To me it's like a very hard and serious, but rewarding game of world cup soccer, in which very few points are often scored, but they mean everything.
There are indeed a few advantages for me as a cabdriver in staggering my shift. I don't have to wait in a heavy pool of hacks at the garage to be assigned a cab. I don't have to wait in line at the end of my shift to collect the cash from my credit transactions. I don't have to use the air conditioning in my cab because by the time it gets real hot midday I'm turning it in. I don't have to stress in afternoon traffic jams about trying to get the cab back on time. I get several hours of quiet, congestion-free traffic flow at the start of my shift. In essence, my shift is what you could call a hybrid of both night and day shifts. I really love it.