Transport for London says:
There is an ongoing programme of initiatives to improve the safety of the Rotherhithe Tunnel. The tunnel is now over 100 years old and was never built to carry the current level of daily traffic.
As part of these initiatives, in March 2012 we put up a 6’6″ (1.98 metres) width restriction sign at the Rotherhithe Tunnel entrances.
The narrowing of the width restrictions on both northbound and southbound approaches to the tunnel will significantly reduce the risk of vehicle collisions, spillage of flammable materials, and fires in the tunnel.
Physical width restriction barriers will be introduced at the tunnel entrances, and enforcement will begin on Tuesday 19 March 2013. Police Community Support Officers (PCSO) and police officers will be in attendance during the first week.
You will only be able to use the tunnel if you have a car, or car-derived van, less than 6’6″ (1.98 metres) in width.
The affected vehicles (an average of 103 per day out of 33,000, or 0.3%) will be required to use Tower Bridge or the Blackwall Tunnel, both of which are outside of the Congestion Charging zone, to complete their journey.