It is clear that electronic road pricing (ERP) remains the cornerstone of the government's policy to control congestion on Singapore's roads.

In principle, this is right: it makes more sense to tax actual road usage rather than car ownership, and the shift in the emphasis of policy over recent years in this direction is appropriate.

However, there remain legitimate questions over the pace and timing of the expansion of the ERP system and its charges.

After 10 years of ERP, the system's effectiveness is still a source of debate. Last week, the government announced another round of ERP changes.

There will be five new ERP gantries and increases in electronic road pricing rates from July 7, aimed at easing congestion in the city centre. They will join 32 existing gantries in the CBD. With the five new ERP gantries, the island will have a total of 65.

There was also a revision of the ERP rate structure, which involves increases in the base charge and increments.

Much of the focus was on the new gantries and the higher charges, but what also attracted attention was the explanation the Land Transport Authority (LTA) gave in revising the ERP rate structure.

The LTA said it is doing so because motorists have become 'less sensitive' to the current rate structure, which has remained unchanged since 1998. This highlights the fundamental problem with ERP. Asked a letter writer to BT: What if motorists remain insensitive to the revised structure?

This is quite plausible. Having won the right to own a car and having paid some of the highest car prices in the world to do so, it would stand to reason that Singapore motorists would be less sensitive to the cost of using roads than motorists elsewhere.

Addressing this point, Minister Mentor Lee Kuan Yew, while speaking at an Institute of Policy Studies seminar earlier this year, acknowledged that the government had got its sums wrong when it implemented ERP. 'Our prediction of consumer behaviour was not quite right,' he said.

The LTA has defended ERP, saying it benefits business by smoothening traffic flow. It also said the intention is not to stop motorists from using their cars, but only to modify their travel patterns.

As a policy objective, this is right. But the fact that motorists have been insensitive to higher ERP charges should give pause for thought. Is simply raising charges and erecting more gantries the right answer? Or does the answer lie in significant and tangible improvements to the public transport system? Probably both.

However, there are questions of timing, pace and sequencing. Raising ERP charges would be most effective in terms of inducing behavioural changes when motorists have a real choice.

Under the Transport Review, new public transport initiatives are to be implemented over the next decade. It is only after these are well advanced that introducing more ERP gantries and higher charges would be most effective in terms of reducing congestion.