Mmmm.... if it is only a couple of hundred spaces at most, out of many thousands, then it doesn't seem worth the grief for a zoo with Chester's income. They could make more money by producing a selling a guide-book!
My guess would be that is is a 'water-tester', and if tolerated will be gradually rolled out over more and more of the car-parks. Much easier to regularly increase it little by little, than to introduce in the first place.
I believe it's 4% of the parking spaces available.