Drivers approaching roadworks are being urged to take extra care after a sharp rise in road-worker deaths this year.
The Highways Agency is urging drivers to concentrate
The Highways Agency said people breaking the speed limit represented the biggest danger.
Road maintenance companies say dangerous driving is on the increase, and they are installing special speed cameras to monitor the problem.
Four workers have died in the first half of 2005 - compared with just one death in the whole of 2004.
Highways Agency traffic operations director Derek Turner called workers on motorways and major 'A' roads a "vulnerable group".
4 deaths, 5 serious injuries
1 death, 7 serious injuries
2 deaths, 10 serious injuries
He said: "Everyone is entitled to a safe workplace, yet road workers risk death and injury at work every day [and night] making sure our roads remain safe and in good condition.
"We appeal to drivers to slow down when approaching roadworks and obey the signs and speed limits which are there to ensure the safety of workers and drivers."
To tackle the problem, contractors are installing the latest speed cameras to monitor average speeds over a whole section of motorway rather than at individual locations.
The Highways Agency says staff are trained to work on the roads - but motorists need to train themselves to take more care through roadworks.
The agency launched a Safety at Roadworks campaign in 2001, advising drivers to keep within the speed limit, concentrate on the road, observe road signs and be alert for works traffic.