Cars don't accidentally drift in the UK - they're controlled by drivers. Drivers drift.It may not be hard, but if cars accidentally drift 5mph over the speed limit in many areas (which is easily done) it's not dangerous either.
If the speed limit for an area is 30mph, people should not be doing 35mph just because it's downhill. In fact, the fact that it's downhill should mean doing a lower speed than on the flat as it'll increase the stopping distance.
If someone doesn't want to get fined for doing 35mph downhill in a 30mph area, they can see the hill and think oh ok I'll brake a bit to keep under the limit.
Most speed camera operations, both fixed and mobile, are determined on the basis of a safety review. We tried to get speed reduction measures in our village in Scotland (mainly speed humps, but cameras would also be acceptable), but the police spent a whole hour talking us through their framework for deciding where fixed and mobile speed cameras go, to basically tell us that there wasn't enough evidence to support a speed camera operation here.It would have far more of an impact on safety if they targeted the areas where it is actually dangerous to exceed the limit (and where some drivers exceed it by a good margin), and the idiots overtaking when there are solid lines down the middle for good reason.
Given the speed camera locations are determined by highways authorities (either National Highways, Transport Scotland, TfL, or local authorities) and the police, and they receive a tiny proportion of the revenue to cover the cost of administration and maintenance, I highly doubt this.They like to claim that the cameras are 'safety cameras', but actually most of them function as revenue cameras. If they were actually about safety that would rule out most of the favourite locations for the van.
Also if it was purely a revenue thing, we wouldn't bother offering speed awareness courses, which are basically the same price but the fees go to the course providers to run the full day course. Scotland was going to introduce these but in fact didn't because they couldn't afford the £250k capex to start the scheme precisely because they receive so little from the speeding revenue.