Emergency bottled water was handed to drivers stuck on the M25 in miles of traffic jams in temperatures of 26C.
All lanes were closed between junctions eight and 10, with traffic halted from junctions six to 12, after an accident.
Emergency services closed the M25 after a lorry caught fire in the Surrey stretch at 1000 BST on Friday. The driver was not seriously hurt.
The clockwise carriageway reopened at 1545 BST, with the anti-clockwise carriageway opening later.
Police said their main priority was the welfare of people stuck in traffic.
Clouds of smoke
The Surrey Police helicopter flew in extra supplies of water which were distributed to motorists.
Diversions were in place and police advised motorists to avoid the area and find alternative routes where possible.
Schools near the accident at Leatherhead were advised to close windows and doors after the fire produced large clouds of smoke.
Surrey Ambulance said it had paramedics on standby in case motorists fell ill in their cars.
Three people were treated at the roadside suffering from the effects of the heat.
"Because of the hot weather people are more likely to get ill in their car if they are sitting around for long periods," said a spokeswoman.
"We have moved crews into positions where they can get onto the motorway more quickly."