Drivers blasted the “chaos” caused by a new trail marathon.
The event was part of Brighton Marathon Weekend, and one driver said he was stuck in traffic for nearly an hour.
Up to 1,500 runners were due to start in Stanmer Park for the first Brighton Trail Marathon before finishing at Hove Lawns.
The worst delays hit Falmer Road and nearby parts of Saltdean and Woodingdean, residents said.
The three fastest men in the trail marathon (Image: Alex Shute)
A Brighton Marathon Weekend spokesperson said they are aware of reports of longer than anticipated traffic hold-ups and are investigating.
Lily Wright wrote on social media: “The backlog caused by stopping the traffic every two cars was a nightmare.”
Many residents said they were not given enough warning about the extra traffic.
Mike Sexton said: “I knew nothing about this until I saw them running down Bazehill Road.
“It has caused significant traffic issues as the A259 is closed to Brighton, so all of that traffic is trying to get through Falmer Road and now this dumb plan has their traffic marshals stopping the traffic on Falmer Road.
“No idea which numpties were responsible for this but they did not do well.”
Ross Keeley said the journey from Saltdean to Woodingdean took nearly an hour, and called it “chaos.”
He said: “I’m all for the marathon, but I don’t think it was a good idea to then have traffic management on a diverted route.”
Other residents said shutting a route already carrying diverted traffic caused gridlock.
But some residents defended the organisers and praised volunteers for keeping runners safe.
Jo Shaw said: “The ‘high viz’ guys did an absolutely brilliant job of keeping the runners safe.
“It was clearly advertised the event was today and ideally, we all adapt for the day and leave a bit more time if travelling by car.”
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The Brighton Marathon Weekend spokesperson said: “The Brighton Trail Marathon was a new inclusion of the Brighton Marathon Weekend this year.
“The route crossed Falmer Road, where stewards briefly stopped traffic so runners could get across safely.
“We are aware of reports of longer than anticipated traffic hold-ups, which we are currently investigating.
“The 2026 Brighton Marathon Weekend was our biggest yet with more than 20,000 people taking part in a series of events.
“As a result of the size and scope of the event we hope to conclude this investigation by the end of the week.
“We hugely appreciate the support of local residents in helping make the weekend such a success, helping to raise millions of pounds for charity and good causes.