Nobody knows how many people will visit the West Country for the best views of the 11 August eclipse but an extra one million visitors are expected in Cornwall.
The agency urged eclipse travellers to plan journeys to avoid peak times. It stressed that drivers all over Britain will experience darkness - even though only part of southern Cornwall will see a 100 per cent eclipse of the sun on the morning of 11 August, weather permitting.
Extra information on eclipse travel will be available from the Highways Agency information line and Internet site from 30 July until 16 August.
David Wright, the agency's manager for Cornwall and South Devon, said the powers would be used as a "last resort if the public safety is endangered. We have done all we can to spread the load by encouraging people to plan well ahead," he said. "Even so I urge people not to consider travelling into the West Country on the day of the eclipse, to avoid disappointment."
Mr Wright said that to predict how many people will travel to Cornwall was "virtually impossible" but the tourist board estimated about one million potential travellers.
An enhanced traffic control centre in Exeter with six closed-circuit television cameras will monitor roads at strategic points. Roadworks on main routes have been suspended.
One-way systems around the West Country and strict limits on traffic allowed into the area were also considered, but rejected by the agency. It wants to maximise the capacity of roads into the South- West and calculations show that hundreds of thousands of extra cars could be accommodated, if travel is staggered over a 24-hour period.
An agency leaflet, Don't Be Left In The Dark, has been distributed to motorway service areas and other outlets. This shows how motorists can plan for eclipse trips. "We are relying on people taking our messages seriously. If they are ignored then there will be problems," Mr Wright added.
He conceded that, whatever happens on 11 August, the agency will be "blamed for something. If we are too effective with our messages and the holiday industry in the South-West suffers we will get blamed for that as well."
The Automobile Association backed the agency's advice and urged eclipse- spotters to book accommodation and plan their journey before travelling. An AA spokesman said: "Thousands of people driving around and looking for hotels, B&Bs or campsites at the last minute will make road conditions even worse. The key to preventing major traffic traumas this summer is plenty of forward planning."Reuse content