The gantries are in Upper Bukit Timah Road (outside Hume Park), Toa Payoh Lorong 6, Upper Boon Keng Road, Kallang Bahru Road and Geylang Bahru Road.

The Land Transport Authority (LTA) said these gantries will be switched on when traffic flow falls below 'optimal speeds' - defined as 45kmh to 65kmh for expressways and 20kmh to 30kmh for non-expressways. Sources expect the ranges to be raised this year, which means Electronic Road Pricing (ERP) could be implemented on more roads - even in the evening.

Some residents are already voicing concern over why the new gantries are in their neighbourhoods.

The LTA spokesman said the new gantry plugs a gap in an outer ERP cordon that seals off non-expressway routes into the city. He said if gantries were at exit points of Toa Payoh, more residents in the estate would be affected.

But the spokesman added that 'the new gantries are built because traffic conditions on these identified roads may soon deteriorate below the optimal speed range'.

Read the full story in Saturday's edition of The Straits Times.