We had one of those at work to monitor drivers speeds, it displayed your speed and if you was over the 20 limit you got a sad face as well, instead of a smiley face. It was also capable of recording the speed and at what time.
Well I always looked forward to it being left out one night so I could "borrow" it and for a couple of nights it was. So whilst out on patrol I positioned it so I could drive as fast as I could past it, but it was not that sucesfull, but I then discovered that if it was on the passengers seat facing forward it would detect everything from lamp posts to tree's and parked cars to lorries on the vehicle park. What a hoot I had for a couple of nights racing round at work, whilst it flashed constantly with high speeds and a sad face.
Makes me wonder what they thought when they downloaded all the info, a few hundred cars all doing way over the limit for an hour, one behind the other and at 0300 in the morning when normally there is just one or two vehicles. Oddly enough they stopped using it just afterwards and got another one which was too big to put in a car, probably thought it had gone haywire.