That is according to a Land Transport Authority study.

In October 2005, ERP gantries in Orchard Road started operating on Saturdays. ERP hours on weekdays were also extended from 7pm to 8pm.

But in the year after the changes, sales went up by 9per cent, more than double the 4per cent growth the year before.

The figure is also higher than the growth rate for businesses outside the Central Business District during the same period, Transport Minister Raymond Lim said yesterday.

He cited the figures in response to Ms Irene Ng (Tampines GRC), who had asked about the social and economic impact of more ERP gantries and longer operating hours.

Mr Lim noted that many factors could affect businesses.

'While retailers within ERP zones have given feedback that ERP is one of the reasons their businesses have been affected, they recognise that their business outside ERP hours has also been poor,' he added.

Mr Lim also assured the House that ERP gantries are not erected pre-emptively.

Rather, traffic situations along congested stretches of roads are studied over several quarters before any gantries go up.

The five new gantries along the Singapore River, for example, were put up after complaints received, over time, about slow traffic in the area.

Another MP, Mr Zaqy Mohamad (Hong Kah GRC), wanted the restriction on the driving of weekend cars on Saturdays removed.

Currently, off-peak cars can be driven only from 3pm onwards on Saturdays. Those who wish to drive them before that time must display a $20 day licence.

Senior Parliamentary Secretary for Transport Teo Ser Luck said a change to usage restrictions for off-peak cars would mean changing the tax concessions for such cars as well.

'However, not all off-peak car owners may agree to a reduction in tax concessions in exchange for such additional benefits,' he said.