The number of drivers unable to pay the toll at the Dartford Crossing between Kent and Essex has nearly tripled in three years.
Last year, 39,996 people were issued with a debt ticket for not having enough cash when they reached the crossing, compared with 15,216 in 2010.
Of those given tickets, 1,336 never paid up compared with 639 in 2010.
Highways Agency spokesman Nigel Gray said the amount of uncollected debt was "absolutely minuscule".
The non-payment figures were obtained via a Freedom of Information request by BBC Radio Kent.
The Dartford toll is £2 each way for cars and £2.50 for two-axle goods vehicles.
It rose from £1.50 for cars and £2 for lorries in October 2012.
UK car drivers with insufficient cash must produce ID to be allowed to continue their journey and must pay the ticket within seven days.
Commercial vehicles and foreign drivers are refused permission to use the crossing and must turn back if they don't have the money.
Mr Gray said the increase in people unable to pay may be connected to the rise in the charge, but the percentage of debt never collected was less than 0.0025%.
"To put it into context, in the last four years some 212 million people have used the crossing," he said.
"Out of those 212 million there are just over 5,000 debts that remained uncollected.
"Every year we are collecting over £70m."
The Dartford to Thurrock crossing consists of the Queen Elizabeth II Bridge and two tunnels under the River Thames.
The barriers and toll booths are due to be removed at the crossing from October, when a free-flow charging system comes into operation.
Drivers will be recorded via number-plate recognition cameras and payment will be made by phone, text, online or in shops or by using pre-paid accounts.