Tax cameras on the M4

Does anyone know anything about some new cameras on the M4 designed to collect the governments latest stealth tax - the SPEED TAX ??

Where have you been it was on the local news a few weeks back for a couple of days. Between junction 18 (bath) & 15 I think, vans on the bridges above on both sides. 79 is the limit

Thought it was jct 14. Swines!