I looked at this product and saw a vent for my driving frustrations.
Looks easily mountable onto parcel shelf to provide an outlet to slow/middle lane/blind/inconsiderate/non-indicating/yellow box filling drivers.
Perhaps not the cheapest find but it was the only reputable seller i could so that mods wouldn't delete the post.
It can be both fun and informative and can be powered by batteries or power supply.
* Program letters, numbers and symbols into any message you like
* Display up to 60 letters in total when programmed fully
* Simple keyboard message input
* 26 pre-programmed messages for added enjoyment
* Built-in digital clock function
* Suitable for wall hanging or self standing with its removable stand
* 3 different brightness levels
Not sure what the law would have to say, but it seems the same as those signs that saw "queue caution" to me. (admittedly they are not on your parcel shelf, nor in your car.)
There are vouchers for maplin over £75 minimum spend so if your buying other stuff don't forget to check those out