The speed limit for the Kallang-Paya Lebar Expressway (KPE) is set to go up from 70kmh to 80kmh as early as next month - six years after the largely underground highway opened.
The Straits Times understands that the limit will be revised to be in line with the limit for the Marina Coastal Expressway (MCE), which opens at the end of the year. The KPE joins the MCE, which also joins the Ayer Rajah Expressway.
New speed-limit signs, covered in black plastic wrappers, have been placed in the tunnels, checks revealed. But certain stretches with sharper curvatures may retain the 70kmh limit.
Speed limits on other expressways are mostly 90kmh.
Associate Professor Wong Yiik Diew, who teaches road transportation at Nanyang Technological University, said the revision may be to align the speed limits of the KPE and MCE. The two highways "may appear disjointed if there were two different speed limits", he added.
An academic paper Prof Wong co-wrote with PhD student Yeung Jian Sheng found that most tunnel accidents in Singapore happen at the entrance or exit. One factor cited was the different speed limits between the surface road and tunnel section.
But the paper found the accident rate in tunnels was on the whole lower than the rate on surface highways.
The KPE's speed limit is enforced largely by laser cameras, which detected 29,404 speeding violations last year. The number represented 12 per cent of speeding offences islandwide over the period - despite the KPE tunnels making up only 0.5 per cent of Singapore's road network.
Retired traffic engineer Joseph Yee said reviewing the speed limit of the KPE "is not a bad thing".
"Seventy is a bit slow, especially when long stretches are straight," he said.
Previous speed revisions were also made for the Central Expressway tunnels and the Ayer Rajah Expressway viaducts, where limits were raised from 70kmh to 80kmh.
"As you develop a higher confidence level over the years, you raise the limit," Mr Yee said.
Regular users of the KPE welcomed the new speed limit, saying traffic in the tunnels is often not heavy, and does not warrant such a conservative cap.
"It should be raised to at least 80kmh because I feel the traffic is too slow for off-peak-hour usage," said bank officer Rongwei Ng, 30.
Army officer Nicholas Chng, 34, agreed, saying: "I have seen some drivers brake really hard only near the speed cameras to prevent getting caught."
Additional reporting by Rachel Tan