The busiest times are expected to be between 11am and 4pm on Sunday and 4pm to 6pm on Monday, RAC motoring group said.
In total, the RAC estimates some 20.7m cars will be using major roads for leisure trips between Thursday and the end of the long weekend on Monday.
RAC Traffic Watch spokesman Rod Dennis said: "Drivers can expect some queues, especially on the busiest routes to the coasts, Lake District and major cities, but by planning ahead and looking at alternative routes they can help alleviate some of the stress that can be associated with long car journeys."
Meanwhile, the M20 in Kent has been closed after a bridge collapsed due to a lorry hitting it. It is expected to be closed until at least tomorrow with severe travel disruption.
The motorway is the main route to the Port of Dover and the Channel Tunnel for people travelling to France.
Speaking of getting away, travel organisation Abta reckons some 2m Britons will be jetting off between Friday and Monday.
Mark Tanzer, ABTA chief executive, said: “This weekend is the traditional curtain closer for the peak summer months and it is always a very busy weekend for travel, with millions taking advantage of the long weekend to head off overseas. With the roads predicted to be extremely busy, holidaymakers should make sure that they leave plenty of time to get to their port of departure.”