Research by the RAC found that 4pm to 7pm is expected to be the busiest period on the roads as motorists embarking on pleasure trips battle for space with regular commuters.
A total of 8.5 million leisure journeys are expected on the road between Friday and Monday.
The travel plans of more than 2,500 UK motorists were analysed for the research.
RAC traffic spokesman Rod Dennis said: “While wall-to-wall sunshine might not be on the cards for everyone this bank holiday weekend, many will still be keen to make the most of the early May warmth by escaping in the car to spend time with family or friends.
“While motorists have told us about their firm travel plans as part of our research, if we see things really warm up by the weekend it’s likely to mean millions more cars on the road, which is bound to cause queues on popular routes.
“We urge all drivers to give their cars a quick once over before they set off, especially if they are planning on driving any great distance.”
A number of rail services will be disrupted by engineering work.
Government-owned Network Rail will carry out more than 820 projects during the two bank holiday weekends in May.
The organisation says it schedules work for bank holidays as there are fewer passengers than normal on those days.
Significant work between Glasgow and Edinburgh means East Coast services will start and terminate at Edinburgh while West Coast trains will not run north of Lancaster/Oxenholme Lake District on Sunday.
Network Rail chief executive Mark Carne said: “Thousands of rail workers will be working round the clock to deliver crucial upgrades to the rail network as part of our Railway Upgrade Plan.
“This huge investment programme will provide faster, better services and help relieve over-crowding to respond to the huge growth on Britain’s railways.
“While most of the network is open for business as usual, some routes are heavily affected and so we strongly advise passengers to plan ahead this May.”