The agency said on Friday workers "found a further crack in a sensitive location which requires us to keep the viaduct closed until the repair is complete".

BBC: M4 closure: Drivers told to stay away

WifiSLAM, which is roughly two years old, is among a slew of technology companies trying to crack the problem of how to detect a mobile-phone user's location when the user is indoors and traditional GPS technology doesn't work.

WSJ: Apple Buys Indoor GPS Specialist

These were the direct ancestors of the "co-location" and "direct data feeds" that have enabled high-frequency traders to get first crack at market prices on today's stock exchanges.

WSJ: New Twist Is Added to an Age-Old Saga