THREE years after submitting proposals and technical applications to the Government Transport Department, Transportation Displays International (TDI) has gained approval for taxi back advertising. Approval means that Hongkong taxis will now feature light boxes on their roofs, panels on the doors and a rear spoiler 10 inches deep by the width of the car boot for advertising purposes. The messages can be seen by pedestrians and following cars. The licence was not automatically forthcoming, despite TDI's current work in promoting advertising on taxis. ''We had a lot of trouble getting a licence,'' said TDI managing director Ms Angela Lau. ''Objections were [made about] the height of the panel which sits on the boot of the car - whether it would obstruct the driver's rear view or hinder reversing,'' she said. Government reticence to approve another licence for on the road advertising is hardly surprising given its reaction to the mobile billboards medium which it banned last July. Critics of the billboards said they were potential road hazards, representing traffic problems to the police and adding to congestion and air pollution. TDI must be relieved to have gained approval for the new medium, given government sensitivity over mobile advertising. The company began advocating taxis as a means of advertising about two years ago when taxi-top light boxes and side panels on doors were introduced to Hongkong taxis. At that time, there was a choice of taxi tops or backs and the 8,000 members of Hongkong's four taxi associations opted for taxi tops. TDI invested more than $1 million into developing suitable light boxes which ended up on 500 taxis travelling throughout Hongkong and the New Territories. Unfortunately, market approval was slow and the varying sizes and shapes of light boxes gave the medium no common factors. ''We lost our investment on taxi tops so decided to do something else,'' said Ms Lau. ''The popularity of taxi back advertising in Australia caused us to think it might succeed here, so we submitted an application to the Transport Department and here we are with government approval three years later and are set to launch in May,'' she said. The diversity of taxi vehicles in Hongkong meant that TDI had to design taxi back panels for each type of car. The government approved the design for the 1993 Nissan and Toyota Crown five-seater models, but turned down the four-seater taxi design, since the vehicles are being phased out. Advertising agencies have been reluctant so far to commit clients to the medium, because they want to see the actual product before spending dollars. Shell Oil, however, may use it for the launch of a new lubricant and a soft drink client is also understood to be considering taxi backs. In a launch package offer, TDI will absorb all production costs for the first 200 vehicles booked - a cost of about $100 per vehicle. The minimum booking is 50 taxis at $1,500 per taxi per month. With a minimum booking period of six months, advertisers are looking at a basic cost of $450,000. Renting door panels costs $300 per month per taxi but the production costs are higher because each panel is a different size.