More than three-quarters of Singapore's 74 electronic road pricing (ERP) gantries have not had their rates changed in the last three years, Senior Minister of State for Transport Josephine Teo said in Parliament yesterday.
Of those that saw changes, only eight showed a discernible upward trend in rates, she added.
But it reduced congestion: Speeds rose an average of 7 per cent per dollar increase, she noted of the eight gantries.
"As for the majority of the gantries which did not need a rate change... a balance has been achieved, with the ERP rate sustaining a traffic speed in the optimal range," she said.
Mrs Teo gave these figures when replying to Mr Baey Yam Keng (Tampines GRC), who had asked for proof that the ERP system was effective.
She said: "The effectiveness of ERP... is not evidenced by the number of gantries that see rate changes, but from the changes in traffic speeds that result from each rate change."
Mr Baey also asked if rate revisions - which are made quarterly - had made it too daunting for motorists to remember the prevailing charges on the road.
Mrs Teo said rates are changed only when average speeds fall outside the "optimal range". The optimal range for expressways is 45kmh to 65kmh, and for arterial roads, 20kmh to 30kmh.
But she acknowledged that rate changes may elude motorists.
"Speaking as a motorist myself, I have to confess that it is probably true of many motorists that even when rate changes are announced in advance through the media, we don't always pay attention," she said.
"Very often, we pay attention when our in-vehicle unit goes beep and we look at the number and it looks different from the last time we were on the road."
It is then that drivers decide whether to change their travelling patterns. Some may still feel the time saved is worth the higher rates, she said.
"But each time there is a rate change, we do notice that there are certain drivers who have changed their travelling pattern."
Mrs Teo said the satellite-based ERP system, which the Land Transport Authority is working on, will be a "fairer" system.
Likely to be in place by 2020, the system offers "the flexibility of charging drivers according to the distance they travel".
"This is an inherently fairer system as those who contribute more to congestion will pay more," Mrs Teo said.
"The incentive for these drivers to change their time, route or mode of travel would thus be stronger."