* Cab size now limited by law
* Changes take time, but outline regulations mark a start
* Streamlining on market in the United States
* Proposals allow megatrucks to cross some borders
BRUSSELS, April 15 (Reuters) - European trucks of the future should have longer, rounded, more aerodynamic cabs to save fuel and cut down on accidents, and adopt some innovations already on the road in the United States.
The European Commission estimates that its new rules regarding trucks, being outlined on Monday, could save around 5,000 euros ($6,600) per year in fuel costs.
They also would cut emissions by 7-to-10 percent for the typical truck, which covers 100,000 km (60,000 miles) annually. In addition, the proposals can help to save the lives of some 300-to-500 cyclists and pedestrians killed in Europe each year in accidents involving trucks.
"A brick is the least aerodynamic object you can imagine and so we are going to improve the shape of the lorries on our roads," EU Transport Commissioner Siim Kallas said in a statement.
Campaign groups representing those killed in collisions with heavy goods vehicles - whose angular, brick-shaped cabs impair driver visibility - have spent years demanding change.
Jeannot Mersch, president of the European Federation of Road Traffic Victims, called for enforcement as soon as possible.
"Lorries have an infamous reputation when it comes to road safety, and rightly so," he said in a statement. "Currently, a frontal crash with a lorry is like hitting a brick wall."
The EU process of securing endorsement of member states and parliament can take around 18 months and often dilutes the force of Commission proposals as industry lobbies against them.
But discussion to relax existing law that limits the length of lorry cabs can begin immediately and give manufacturers, with long planning cycles, guidance on future requirements.
A Commission official, speaking on condition of anonymity, said change should happen quickly. Already, so-called "boat tails" - retractable flaps, which can be added to the back of vehicles - are in use in the United States, he said.
The cost was around 2,000 to 3,000 euros ($2,600-$3,900), but that is recovered in one or two years through fuel savings.
"Some manufacturers are very keen on what we are doing. Others, however, have recently spent a lot of money making the current generation of trucks, which might be rendered obsolete a little quicker," another Commission official said, also speaking on condition of anonymity.
He declined to name truck makers and none was immediately available for comment.
Environment campaign group Transport & Environment said the rules were "a turning point".
"The proposal is a small step towards freight transport fit for the 21st century," T&E policy officer, William Todts, said.
His one criticism was that the Commission proposes to allow 25-m (yard-) long trucks, to travel between adjoining member states provided both countries allow vehicles of that length.
"The Commission has opened the door to cross-border use of megatrucks in Europe without appropriate guarantees for both citizen safety and environmental protection. We need better trucks, not bigger trucks," Todts said. ($1 = 0.7635 euros)