Up to 8,000 drivers a day are failing to pay the Dartford Crossing tolls since the barriers were done away with a month ago.
A new electronic scheme is now in operation with 85% paying the £2.50 charge for cars. It is similar to Congestion Charging in London.
The first penalty notices have just been sent out but initially offenders will have two weeks to pay the charge before they are fined.
Meanwhile the Highways Agency say there have been big savings in journey times with many trips on average speeded up by nine minutes.
There is still work needed to remove barriers and it is expected to be complete in the Spring.
Our transport correspondent Mike Pearse speaks to Nigel Gray from the Highways Agency.