The Highways Agency said that the full benefit of the changes will only be felt when the toll booths are removed.
Dart Charge Project Director Nigel Gray said: "We will get underway on this work at the same time that the new payment arrangements begin. It's complicated work - especially on the northbound carriageway, where we need to protect the tunnels - one of which was built in the 1960s and has a lower height restriction - from over-height vehicles.
"We’ll be completely redesigning the approach to the tunnels to detect and turn around these vehicles and other vehicles not suitable for the tunnels, using a system of signals, barriers and extra lanes, while keeping all other traffic flowing as much as possible.”
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