IN the letter headlined ''Drivers need adequate warning'' (South China Morning Post, June 16), the maintenance vehicles in question are deployed for picking up refuse obstacles from the carriageway to ensure safety on high-speed roads. These dangerous obstacles include spare tyres and various goods that fall off from vehicles that require immediate removal. Each of the maintenance vehicles, when working on the highway, is mounted with a flashing arrow sign of 1.8 metres by 0.9 metres rising 3.4 metres above road level as a warning to motorists. In addition, this vehicle is followed by another escort vehicle which also carries a similar and conspicuous flashing arrow sign to provide ample advance warning to motorists. Previously, the flashing arrow sign used to be mounted on a trailer on tow behind the works vehicle. However, due to its small size, the trailer could only carry a flashing sign of 1.5 metres by 0.75 metres mounted with its top no more than 2.3 metres above road level, otherwise it would topple on dragging by the works vehicle. Hence, it was less conspicuous than the bigger sign that's now being used on the escort vehicle. As for reporting highway accidents, emergency telephones are installed for use by motorists along the verges of the highway at intervals of about 500 metres. FELIX LEUNG for Chief Highway Engineer (D & M)/N.T. (District and Maintenance) New Territories