The Dartford Tunnel and Queen Elizabeth II bridge cross the River Thames between Dartford and Thurrock.
They form a link in the M25 motorway, carrying about 150,000 vehicles a day.
Actually, pedantically, the crossings are not part of the M25 at all. The orbital motorway joins each end of the A282.
I’ve said it before, and I’ll say it again: remove the tolls and the congestion will almost miraculously dissipate.