Passing zones always presented interesting situations for traffic enforcement. There were many times when I would find one driver in the right lane traveling at or near the speed limit and another passing by in the left at a speed significantly in excess of the speed limit. On stopping the speeder I would often hear about how they had been forced to travel behind the slower vehicle, which had been going well under the limit, for great distances and how that slow driver sped up on reaching the passing lane.
My difficulty was that the passing lanes were good opportunities to travel at the speed limit compared to the highway leading up to them. Experience had taught me that if I applied my speed "allowance" for drivers over the limit to those under the limit and watched the advisory speed signs, speeders were a dime a dozen and truly slow drivers were like hen's teeth.
"Isn't there a law about increasing your speed while being passed?" I was often asked. Yes there is. Except where passing on the right is allowed, a driver being passed must not increase their speed until they are completely passed by the overtaking vehicle.
Passing zones permit passing on the right because there are at least two adjacent lanes for the same direction of travel. So, the previously slow driver is allowed to speed up to the limit in the passing zone. If you have to exceed the limit in order to pass them, you take your chances with law enforcement.