mini-moan: define 'definition'

Today while travelling down the A299, oh how I miss driving down the A299, I noticed a sign moving past me at exactly 70 miles per hour, or more precisely a sign while my motorised vehicle was moving at exactly 70mph for that is the speed limit of the A299. Anyway the sign read 'Prohibited Traffic [this way]'. What? If you want the traffic to go down the road then it can't be 'Prohibited' for the word means;

Formally forbid (something) by law, rule, or other authority.

Or perhaps it's a new way for the government to raise money and just down the road, out of sight, are a crack team of police ready to capture any individuals who has chosen to ignore the rule prohibiting them and slap them with a MASSIVE FINE, certainly an imaginative way to solve the budget deficit.