The worst traffic jams of the year are expected this afternoon when the start of half-term causes a phenomenon on the roads known as the Black Friday effect.

The RAC and Trafficmaster said that there could be almost double the number of delays experienced during the August Bank Holiday. But, unlike on that holiday and others during the year, the authorities will not stop roadworks to cut the congestion.

Trafficmaster has 7,500 roadside sensors to measure traffic speed on 8,000 miles of motorways and trunk roads and today it is expecting to register more than 8,000 alerts of traffic moving slower than 30mph.

The analysis of alerts on nine key routes since 2003 indicates that there have been more traffic jams during the Friday evening rush hour at the start of the autumn half-term than at any other holiday period. This is because of a rise in commercial traffic in the autumn, an increase in accidents and a combination of darker nights and wetter weather that encourage more drivers to use well-lit major roads. Also, unlike during the spring and summer holidays, fewer families will go abroad.

The Black Friday effect has been increasing for the past three years, with 24 per cent more delays last year than on the same Friday in 2004. Trafficmaster said: “It seems odd to think that a Friday in October is busier than Christmas or Easter, but the Black Friday phenomenon is very real and one that all drivers should be prepared for. Just a minority of motorists replanning their routes or the timing of their journeys could help to make sure that this Friday isn’t the worst of all.”

The worst areas for congestion are expected to include the M25 western and northern sections, the M1 in Hertfordshire, East Midlands and South Yorkshire and the M6 in the West Midlands around Birmingham. Long delays are likely on the A421 near Milton Keynes, the A5 at Dunstable, the A3 at Hindhead, Surrey, the A14 between Huntington and Cambridge and the A720 at Edinburgh.

Edmund King, executive director of the RAC Foundation, said that the Friday evening peak was the worst time for accidents because drivers tended to lose concentration at the end of the working week. “More car users are killed and injured between 4pm and 6pm on a Friday night than at any other time during the week.”

The worst routes have been identified by the two organisations in the UK Congestion Report,which says that the western section of the M25 is Britain’s biggest congestion spot, replacing the M1 in Hertfordshire in the past six months, where a temporary speed limit of 40mph has been raised to 50mph in road-widening works.

The report also compared the average speed of motorway traffic. On the fastest, the A74(M) southbound in Scotland, the average driver broke the limit at 71.5mph. Given that lorries have limiters restricting them to 56mph, this indicates that a significant proportion of car drivers must be speeding at faster than 90mph.

The slowest motorway, at 52.7 mph, was the M8 westbound in Scotland and the eastbound section has the second slowest traffic at 54mph, followed by the M60, 54.7mph anticlockwise and 55.5 clockwise, and the M25 with speeds of 55.7mph anticlockwise and 55.9mph clockwise. The M40 was the second fastest, at 70mph northbound and 69.8mph going south, followed by the M6 toll road, with 69.4mph southbound.

The Highways Agency predicts “severe” delays of more than 30 minutes at two sets of motorway works today: the M1 Junction 31-32 northbound near Sheffield and the M4 Junction 16-17 westbound in Wiltshire. Severe delays can also be expected on the A5036 northbound between the A5038 and A5207 on Merseyside and on the A590 westbound between the A6 and A592 in Cumbria.

“It will be busy because it is the half-term holiday but all roadworks will remain,” a spokeswoman said. “We are treating it as business as usual and making no special provisions. We have no evidence to say it is going to be any different from any other Bank Holiday and will have to wait and see what happens.”

Rugby fans, heading to Paris for the World Cup final, are expected to contribute to congestion on routes to the Channel ports. Strikes involving public transport in France are likely to tempt many to drive rather than fly or travel by Eurostar.

Stores stocking up for Christmas were partly responsible for the extra commercial traffic on the roads this month, the report said. Internet shopping has also contributed to a 39 per cent increase in light van traffic in the past ten years. The Department for Transport is planning to ease congestion on some motorways by allowing the hard shoulder to be used at peak times. Trials on an 11-mile stretch of the M42 southeast of Birmingham have shown that there is no impact on safety.

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