The Tai Lam Tunnel is due to be returned to the government in May next year when its 30-year "build-operate-transfer" franchise will come to an end.

Officials at the Transport and Logistics Bureau will then have one more tool in their bag to redistribute traffic flows across Hong Kong with a view to maximizing the efficiency of the city's road networks, especially in the western part of the city.

Armed with the successful experience of diverting the traffic at the three cross-harbor tunnels, it is very likely that the authorities will arrive at a plan to divert traffic to the Tai Lam Tunnel to ease congestion on the Tuen Mun Road expressway and improve traffic flow on the Tolo Highway.

In a new document submitted to the Legislative Council, the bureau has put forward three probable toll schemes for the tunnel after May next year.

In all scenarios, the tolls will be lowered.

In the first scenario, it is assumed that, for example, the tolls for private cars and taxis would be reduced from HK$58 to a fixed rate of HK$45.

In the second scenario, the fixed rate would be reduced further to HK$35 for private cars and taxis, for example.

In the third, a regime of variable tolls of between HK$20 and HK$45 would be charged.

For example, private cars would be charged a peak rate of HK$45 for the busiest periods of the day and HK$20 to HK$35 for other periods.

In view of the bureau's comment that lowering the toll to HK$35 may lead to too much traffic being diverted to the Tai Lam Tunnel, causing congestion at the tunnel and its approach roads, it is very unlikely that the transport authorities will recommend this in their final report in spite of demands from some lawmakers to keep the tolls as low as is viable.

Unless more scenarios are created later, transport officials are set to choose between a fixed rate of HK$45 for private cars and a system of time-sensitive variable tolls from HK$20 to HK$45. The question is: which is preferred?

While it is forecast that the two schemes would improve traffic flows similarly, charging fixed tolls would be friendly to government finances but unfriendly to motorists.

Imposing a time-sensitive variable toll scheme would generate less revenue for the government but motorists would welcome it.

So which is more important - money or public sentiment? That will be something not for the technocrats but politicians to judge.

Nonetheless, charging a variable scheme is not unprecedented. Since December last year, variable tolls have been introduced for the three cross-harbor tunnels and have turned out to be successful.

Keeping the tolls low will be welcomed by road users.

So-called "B-O-T" projects are not new to Hong Kong as all three cross-harbor tunnels were built by private consortia on such a basis.

Construction of the Tai Lam Tunnel began in March 1995 and it opened to traffic in May 1998 in time for the opening of the Hong Kong International Airport in Chek Lap Kok.

A merit of the "B-O-T" approach is that it leverages on private-sector resources to provide an answer to the public need, especially when the government comes under the constraints of a budget deficit.

Tolls driven by money or public sentiment?