IT will cost motorists $1.50 to use the Bukit Timah Expressway (BKE) between 7.30am and 8am on weekdays from next Monday - up from $1 now.

The electronic road-pricing (ERP) gantry on the BKE between Dairy Farm Road and the Pan-Island Expressway is the only one being tweaked by the Land Transport Authority in its latest review of ERP rates.

The rates at all other gantries remain unchanged - until the next review in late-May, just before the June school holidays.

ERP rates are reviewed once every quarter as welll as just before school holidays. Prices are usually lowered for the latter.

The LTA did not say why it is raising the 7.30am-8am slot for the lone BKE gantry, but ERP rates are usually raised when average traffic speeds fall below the optimal 45kmh to 60kmh range for expressways.

The idea is to spread out demand so as to avoid congestion and achieve better traffic flow overall.

The fact that rates at all other gantries remain unchanged indicate that traffic flow at all ERP-controlled roads has not improved or deteriorated significantly in the last three months.

Miniscule adjustments, however, will be a thing of the past from July, when a new set of criteria for rate movements kick in.

As part of a slew of measures to control congestion and persuade more people to take public transport, ERP increments will be at least $1 each time - double the 50-cent jumps now.

That's not all. A new interpretation of 'optimal speed' will take effect. Instead of taking average speeds as a criterion for ERP rate changes, a more stringent method that ensures than 85 per cent of road users experience the optimal speed range will be applied.

In effect, the two new meaures mean the likelihood of more aggressive rate increases.

They will apply in the CBD and Orchard area from July; and at most other ERP-controlled roads from November.

The remaining handful of outlying gantries will be affected from February next year.