This is the time of year when combines and tractors flock to the roads in considerable numbers. David Cousins asked transport specialist Mike Braithwaite to outline some of the legal stuff you should know about.

Do you take combines or wide implements on to public roads without either notifying the police or getting a Dispensation Order? If you do, you're in good company, since most other farmers probably don't either.

But if the machine or implement in question is more than 3m wide, you're technically committing a road traffic offence by not notifying the police beforehand, says Mike Braithwaite, who specialises in advice on agricultural vehicles on the road. More seriously, if you are involved in an accident, the penalty doled out is likely to be that much greater.

There are two ways of staying legal, he says. One is to notify your local Abnormal Loads Office 24 hours ahead of movement. That might be feasible in theory, but in the rush of harvest (and especially if the weather is changing all the time) it can be difficult and sometimes just plain impossible.

There is a much easier way to do it anyway, points out Mr Braithwaite. "If you have to take a combine or any other wide implement on the road repeatedly, it may be simpler to apply for a Dispensation Order. This lasts for a calendar year and means you don't have to notify the police each time you take the combine on the road," he adds.

"Write a letter to the Chief Constable requesting a dispensation. Outline what equipment over 3m wide (including combines, foragers, power harrows, cultivators, etc) you'll be taking on the road. Photocopy a local map and circle the area you'll be travelling within."

Some police forces will allow a Dispensation Order for combines up to 4.3m wide. Others (particularly in areas with narrow roads like Devon and Cornwall) may restrict their use to combines less than 3.5m wide.

Equally, some police forces may not allow combines to be moved at night and others may put what seems an unreasonably low limit on the mileage. You may want to try to negotiate that figure upwards.

Even with a Dispensation Order you need to comply with the other rules that apply to wide vehicles on the road.

• If the machine or implement is between 3m and 3.5m wide it is good practice (though technically not a legal requirement) to have an escort vehicle.
• If the machine is more than 3.5m wide, however, you are legally obliged to have an escort vehicle.
• This must be a separate vehicle, with an amber flashing beacon, running in front (on a single carriageway) or behind (on a dual carriageway) the wide vehicle. It must also be there solely to perform escort duties, so the tractor pulling the combine header trailer won't do.
• The combine also needs to be properly lit with at least one flashing beacon visible from 360°. Headlamps should be switched on, but work lights should not. Mirrors, lights and indicators, it goes without saying, should be clean and a walkie-talkie link between combine driver and escort vehicle is pretty essential, too.
• If an implement (like a cultivator) exceeds the width of the tractor by more than 305mm on each side, you must fit marker boards.