SOME form of automatic fare increase mechanism must be given to the operator of the Route 3 Highway if private investment is to be attracted to fund the multi-billion project, Secretary for Transport Haider Barma said yesterday. Mr Barma told legislators on the transport panel: ''If we invite people to bid for the project, they want some form of guarantee that they will have a fair return on the investment. Therefore, there must be some kind of mechanism. ''If the administration says that this is the fixed tunnel toll initially, every time you want to increase it, you've got to come back and get approval from the Legco, this to the investor will be a major hurdle.'' Legislators have warned that the controversial Western Harbour Crossing franchise - which included an automatic fare mechanism - should not be a precedent for other projects. Controversy over the franchise started last June. Legislators were angry that the operator was allowed to increase the tunnel toll by $10 whenever its rate of return fell below 15 per cent. ''We are not saying we'll adopt the Western Harbour Crossing model,'' Mr Barma said. ''It's up to the bidders to indicate what they want.'' But legislators said it was inappropriate to encourage bidders to adopt an automatic fare increase mechanism. ''When the Government persuaded us to pass the Western Harbour Crossing franchise last time, it said time was running out and there was only one bidder. Now, this won't happen in the case of Route 3,'' United Democrat legislator Lee Wing-tat said. ''So, why should we follow the example of the Western Harbour Crossing now?'' So far, six consortiums have said they are interested in the bid. Liberal Party legislator Steven Poon Kwok-lim said legislators should work out with the Government their preferred toll increase formula instead of allowing bidders to take the initiative. Mr Barma replied that the Government was ready to consult legislators. He admitted the crux of the bid would be the level of the opening toll and the toll increase mechanism. The $6-billion Route 3 Country Park section, which runs from Au Tau to Ting Kau, is expected to call for tenders by the end of the year. The Government considers it to be the solution to the notorious traffic congestion along the Tuen Mun Highway. Contracting out the management of 13,000 public parking meters to a private operator would speed up the replacement of mechanical meters with electronic models, said Commissioner for Transport Rafael Hui Si-yan. ''Our problem is that we can't find the $70 million to replace all the meters at a stroke,'' he said. Mr Hui said the new arrangement could speed up the efficiency of operating and repairing meters.