Drivers are being urged to ignore satnavs and only follow official diversion routes to prevent causing gridlock during an “unprecedented” closure of the M25 this weekend.
National Highways issued the alert as it prepares to close a five-mile stretch of the motorway in Surrey from 9pm on Friday until 6am on Monday.
The Government-owned company estimates that drivers who follow signs for diversion routes on A-roads will have an hour added to their usual journey times between junctions 10 and 11, which will be shut in both directions.
It is concerned that some may try to find alternative routes via minor roads.
National Highways project lead Jonathan Wade said how well the area copes with the M25 closure will partly depend on whether drivers stick to official diversions.
He told the PA news agency: “How many people are going to take the initiative and try and use satnavs?
“There’s probably a greater risk of congestion by people just doing their own thing and thinking they can perhaps beat the signs and find a shorter or quicker route.
“That will cause further congestion on some of the key junctions so please avoid doing that if at all possible.”
National Highways senior project manager Daniel Kittredge said: “If people move away from diversion routes that we prescribe, it creates additional issues in different parts of the road network.
“The majority of the time that will be local roads, so that really impacts residents in those particular areas.
ADVANCE WARNING: the M25 will be closed both ways between junction 10 and 11 from 9pm Fri 15 March to 6am Mon 18 March.
Only travel this stretch if necessary. Significant delays and disruptions expected.
Check updates:
➡️ https://t.co/YiY0SVelYb pic.twitter.com/sWAD8QR2kR
— National Highways (@NationalHways) March 11, 2024
“That’s why we’re trying to encourage people to not follow the satnav.
“Stick on the prescribed diversion route. It’s going to be more suitable for your journey.”
Drivers are being warned to avoid the area if possible during the closure, which will be the first scheduled daytime all-lanes shutdown on the M25 since it opened in 1986.
National Highways said the action is necessary to enable a bridge to be demolished and a new gantry to be installed.
Modelling carried out while the plans were being developed estimated that drivers would face delays of up to five hours without mitigation measures such as urging drivers to stay away and creating diversion routes.
National Highways believes only around an hour will be added to journeys because of the steps it has taken, which is based on a reduction in traffic of 50%.
But Mr Wade said: “Because it’s so unprecedented, we’ve got nothing to benchmark it against.”
He added: “There’s never been a closure of this nature. We really can’t be certain how many people will heed the messages which we’ve given.
“It’s very difficult to determine right now how effective all our traffic management will be.
“Please don’t travel if you can avoid it.”
The M25 normally carries between 4,000 and 6,000 vehicles in each direction per hour from 10am until 9pm at weekends between junctions 9 and 11.
Four more daytime closures of the motorway will take place up to September.
The project, due to be completed in summer 2025, will increase the number of lanes and make it easier to enter and exit the M25 at junction 10, which is one of the UK’s busiest and most dangerous motorway junctions.