In message , Graeme
writes

IIRC in the original installations each lane was monitored by it's own set of
cameras. The assumption was each set was independent all through the chain.
Not necessarily true but urban myths get taken up very rapidly.

According to the manufacturer of SPECS, writing in 2008 and quoted in
the article in The Register reference up-thread, it was no urban myth,
but simply the case that the cameras were not authorised for use in
situations where a vehicle had changed lanes between check points:

"Until recently, the only HOTA [Home Office Type Approval] available
applied to cars maintaining their lanes. However a new test schedule was
carried out last year, which means that average speed checking can be
applied even where cars change lanes."

In other words, SPECS could technically work out the average speed, even
when the driver changed lanes, but if the speed was above the limit
there would be little chance of a successful prosecution because the
system would not have been deemed to have been used in an approved
manner prior to late 2007.
--
Paul Terry