The RAC warned drivers to expect delays on Friday as commuter traffic will share space with recreational travellers.
But Monday will be the busiest day for leisure trips with 3.4 million such journeys, as motorists head home after the long weekend.
RAC traffic spokesman Rod Dennis said: "With Easter just a fortnight ago, for many people the May Day bank holiday may have come as something of a pleasant surprise.
"Certainly our figures show that many of us will be packing the car up to enjoy a trip to see friends or family, with the vast majority of leisure journeys expected on Bank Holiday Monday itself which suggests day trips and drivers returning after a short break will be responsible for much of the traffic."
He advised anyone travelling a long distance to give their car "a quick once over" before setting off to cut the chance of suffering a breakdown.
Rail passengers are being urged to check for timetable changes as a number of lines will be closed for engineering work.
:: No trains between London Liverpool Street and Shenfield/Ingatestone/Billericay
:: Buses replacing trains between Carlisle and Glasgow Central/Edinburgh
:: No Southern services to or from London Bridge
A Network Rail spokeswoman said: "Rail staff will be working to deliver crucial upgrades to the rail network throughout the bank holiday weekend.
"We don't like inconveniencing people but we have to carry out this work when the railway is quietest, and we know that up to 50% fewer passengers travel by rail over bank holidays.
"The majority of the network is unaffected."