the reason that these weigh bridge are open are to stop lorry/vans and even cars being over loaded.
which when there are,can be dangerous to the driver and other user of the road,
if over its a fine,but not points on your licence,and if you got a hgv leicence then that can be taken away from you.
i know cause i being there and learned the hard way.at the time i was driving a artic,and was 3.5 tons over on a axles weight,and not gross weight,which if it would have been then that another offence.