The estimate comes from the AA and traffic information company Trafficmaster, with both organisations forecasting a busier-than-usual evening rush-hour on Friday and more jams on Saturday.
With cooler, rainier conditions forecast, those travelling by rail will have to contend with engineering work in places, including amendments to West Coast services due to engineering work between London and Rugby in the West Midlands.
On the roads, a series of sporting events, air shows and music festivals are likely to add to congestion.
These are some of the routes which Trafficmaster reckons will be particularly busy:
:: All sections of the M25, especially both ways between junctions 12 and 17 west of London near Heathrow Airport
:: The M1 in Hertfordshire, Bedfordshire and Buckinghamshire
:: The M6 in the Midlands, the M5 in south-west England and the A303 heading west
:: The M4 westbound from west London to Newbury in Berkshire and the section going into Wales
AA patrolman of the year Mark Spowage said: ""Whatever the weather, drivers should be prepared for extra traffic and possible delays on major routes to coastal areas and holiday resorts.
"Planning your journey and avoiding the main bottlenecks could relieve the frustration of sitting in a traffic jam.The roads are likely to be extremely busy and it could take a lot longer to get to your destination than you expect.
Buses will replace trains on a number of rail routes. The West Coast work will mean reduced services between London and Birmingham New Street/Manchester Piccadilly from 8pm on Saturday to noon on Monday.
Engineering work is also taking place between Liverpool South Parkway and Edge Hill with all lines closed over the weekend..
Routes where buses replace trains include between Twyford in Berkshire and Henley in Oxfordshire, Slough to Reading in Berkshire, Dover Priory to Ramsgate in Kent, between Glasgow Central and Whifflet in Scotland, between Knottingley and Goole in Yorkshire, and between Oxford and Bicester in Oxfordshire.
Among routes with amended services at some times over the weekend are Newport to Cardiff Central, Cardiff Central to Fishguard Harbour, Sheffield to Mexborough/Moorthorpe in Yorkshire, and Falkirk Grahamston to Stirling in Scotland.
There will also be an amended service at Colchester in Essex due to platform closures.