POLICE want to get unmarked cars with hidden cameras so they can record some of the mad and dangerous things Hong Kong drivers do on the Tolo Highway. The cars have been on three-month trial in the territory, and police top brass will decide on Monday whether to ask the Government for the $2.5 million needed to buy them. Police Deputy Director of Traffic Jim Walker said: ''I'll certainly be recommending to the meeting that they do.'' He said the main cause of accidents on the expressway was drivers going too fast and getting too close to the vehicle in front. But drivers often refused to accept that their driving had been dangerous, and police had a hard time proving it in court. ''Without the cameras it is one man's word against another's. Drivers in Hong Kong are usually well educated and intelligent, and it can be difficult for magistrates to decide who is in the right.'' If the video evidence was there, this would no longer be a problem, he said.