As a former California Highway Patrol officer we would spot an expired reg sticker due to it's color then run the tags too verify, Previous years the DMV issued four stickers per vehicle, now with three stickers an officer parked in the medium can see the rear left corner side as the car passes by and the officer can see right rear side from traffic lanes.
Yes the color of the reg sticker is what gives the year away, but then the different HOV stickers have different colors too: white, green, and the now expired yellow.
My point being that if that tiny little square in the upper right corner of a license plate is visible from a considerable distance for those trained to look for it, a single small HOV sticker on the right rear bumper should suffice. But if you have to see it from the front, then a small sticker on the left front bumper is fine, and that is still the case with the green stickers. What I don't care for is the number and especially the size of the stickers that all go on the rear of the car. IMHO it's unnecessary.
CHP are also pretty good at spotting those without their seatbelts on, and that can be tough if the seatbelt of the car happens to closely match what the person is wearing. (BTW...fellow 830 PCer here)
Blue Ocean 2012 Leaf SV, lost that 1st bar at 34 months/26,435 miles. Lease returned 2 months later. Final LeafStat figures: 225 Gids, 17.44 kWH, SOC 91.89%, SOH 82.36%, 69.49% HX, 54.57 Ahr, battery temp 61.8 F.
Now driving a 2015 VW eGolf SEL.