TRAVELLERS in France are being warned to be on the look-out for robbers posing as traffic police after a Belgian couple were tricked into handing over pounds 827. The crooks, in plain clothes but driving a car with a flashing light, stopped the Belgians' camper van on a minor road near Nay in south-west France and asked to see their papers. They threatened to take them to the police station unless they handed over all their cash.