Colin Sowman looks at the transition to free-flow charging on the Dartford Crossing, a notorious congestion blackspot on the UK motorway network. The Dartford Crossing, where London’s orbital M25 motorway crosses the lower reaches of the River Thames 32km (20 miles) to the east of Central London, has long been a major source of congestion. Now, to alleviate the congestion caused by some 50 million crossings per year, the Highways Agency has adopted a free-flow charging system - but the Crossing’s location a