Trafficmaster has found that the western part of the M25, between 5pm and 6pm, and between junctions 10 and 21A on Fridays in August, is the most congested in the country. The northern section of the M25, between junctions 21A and 30, is also best avoided, as is the southbound section of the M1, between junctions 12 and 6A.
A Trafficmaster spokesman said: "Fridays within this month are on average busier than a normal Friday. Many people make the most of the weekend during the summer holidays and with holidaymakers mixing with commuter traffic, delays are more frequent and longer."
Other congested roads in the survey include:
* The M4 from London to Bristol and between the Severn Bridge and Cardiff;
* The M62 in West Yorkshire between junction 23 and the M621 at junction 7;
* The M8 in Glasgow between junctions 13 and 25;
* The M6 in the West Midlands between junctions 4 and 10A;
* The M5 Avonmouth to Almondsbury near Bristol.
Trafficmaster also found that 40 per cent of drivers spent an hour a day stuck in traffic and that last year motorists wasted 182 million working days in jams - costing the UK up to £20bn.
Apart from 5pm to 6pm, the worst time for congestion is 4pm to 5pm, followed by 8am to 9am. October is the next-worst month, followed by September, July and November.
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