The opening weekend of the London Olympics in July has all the ingredients to whip up a 'perfect traffic storm', a report analysing expected summer congestion levels in the capital warned on Wednesday.
The report by real-time traffic information providers INRIX predicted that Londoners faced a 33 percent rise in congestion levels while traffic on core network routes would slow to speeds of around 12 miles per hour.
The opening ceremony at the Olympic Stadium on Friday July 27 coincides with the end of school terms and start of one of the busiest holiday periods of the British summer.
The men's cycling road race, with Briton Mark Cavendish an early favourite to take the first home gold, follows on the Saturday on public roads through southwest London and into surrounding counties.
"One of the busiest holiday getaway weekends, the Olympic opening ceremony and the men's road cycling race is the perfect traffic storm," said the report's author Greg Hallsworth.
"Nearly 100,000 ticket-holders are expected at the opening ceremony, with tens of thousands of visitors anticipated at the live sites at Hyde Park and Victoria Park.
"This, combined with thousands of closed roads for the men's road cycling event will undeniably create huge stress on the UK's road networks."
INRIX predicted that journeys with an average travel time of an hour would take at least 12 minutes longer throughout Greater London for the duration of the Games. There would be even longer delays through known traffic hotspots.
"People travelling on the Core Games Network should plan for at least 20 minutes more for their journey," it added.
Rush hour times are likely to start up to 90 minutes earlier than usual during the Games in early August, with morning peak traffic as early as 0530.
However, evening congestion could ease considerably earlier than usual due to people heading to meeting places to watch events on television.
"To minimise gridlocks, we advise commuters to build travel schedules around quieter periods where possible," said Hallsworth.
"Despite this generally being a quieter time of year, Londoners need to be prepared for a huge volume of traffic in the City that they are unlikely to have experienced previously."
Londoners are already unhappy that parts of their crowded road network will be reserved for athletes, officials, media and sponsors using designated 'Olympic Lanes' during the Games.
Organisers want London to be known as the "public transport Games" and have identified transport as one of their big challenges, along with security. Critics fear the system will struggle to cope with the additional demand.
Motorists will not be banned from driving in the centre of London, however.