Following on your response about speed cameras at night, it's worth noting that the police can of course use a laser speed device at night (although arguably forming prior opinion of excess speed is a lot harder at night - which reduces the effective range). However, the vans (usually operated by the safety camera partnerships) need video evidence and having seen the footage from a couple that were obtained at dusk (with all cars coming towards them with headlights on) it's almost impossible to positively identify a vehicle. In general, until now, if there's a camera van at the side of the road at night it's not doing speed enforcement. It's somewhat worrying if they've started such enforcement as the obvious reaction (people braking harshly) is more likely to cause an accident.