Almost 250 people were caught speeding in the first three days since enforcement was introduced on the M4 around Newport.
The variable speed limit covers the stretch from junction 24 (Coldra) to junction 28 (Tredegar Park).
The system was introduced in 2011 but was inactive for five years - fines have only been issued since 10 October.
GoSafe said 62 people were caught on the Monday, 77 on Tuesday with another 101 on Wednesday.
The organisation, which enforces the speed limits, added the enforcement action was clearly indicated on illuminated signs and there was "no excuse" to flout the law.
Prosecutions can be handed out whatever the variable limit is - from 20mph (32km/h) to 70mph (112km/h).