A pregnant wild boar was killed when it rushed onto a stretch of the Bukit Timah Expressway near Mandai Road on Thursday night.
Police said three cars were involved in the accident. No one in the cars was injured.
The Straits Times understands that the wild boar was struck by two cars travelling in the rightmost lane of the expressway. A third car then collided with the second car, which had come to a stop.
It was the second time in a week that a wild animal was killed on the same stretch of expressway by passing traffic.
On Sunday, a wild sambar deer wandered onto this same stretch and was hit by a taxi. The injured driver was taken to hospital, while the deer had to be put down.
Wildlife experts said the animals came to grief as they were trying to cross the expressway to get from one stretch of forest to another, and said the accidents could be linked to recent developments in the area shrinking their habitats.
A report in The Straits Times in March said at least three animals had been killed on the roads since the construction of new wildlife parks began in Mandai in January last year.
A dead Sunda pangolin and leopard cat were found on Mandai Lake Road and a sambar deer on Mandai Road inside the project boundary or within 500m of the works.
Wildlife consultant Subaraj Rajathurai and biology lecturer N. Sivasothi separately noted that construction work had resulted in the loss of foraging areas inside the development area, such as grassland for the sambar deer.
"It is time to take mitigation measures for the Mandai project seriously," Mr Subaraj told The Straits Times. "Not only is it affecting animals, it is also causing humans to get injured."
When queried, project developer Mandai Park Development (MPD) provided a background factsheet of its mitigation measures to facilitate wildlife connectivity and reduce the occurrence of such events.
For instance, it has erected a rope bridge across Mandai Lake Road to help tree-dwelling animals move across more easily, and has plans for another.
It is also putting up hoardings to guide animals towards safe crossings.
MPD noted that the speed limit for most parts of Mandai Lake Road has been reduced to between 20kmh and 40kmh, while speed humps, speed regulating strips, speed signs and speed radar panels were also put in place.
It has reached out to major transport companies - including public buses, taxis and private-hire companies - and travel agents that operate tour buses to raise awareness on existing speed calming measures.
Mr Sivasothi, from the National University of Singapore, said developers tend to look only at their sites but in such developments, it is critical to understand the holistic picture to have effective mitigation.
He said: "Holistic means that you can't just look at Mandai Lake Road, but look at the entire area. If you look at where the development clearance is, and where the roadkill happened, they are not adjacent."
But with foraging ground being developed, animals need to move. "And unfortunately in Singapore you will soon come to an obstacle like a road," he said.
"The very important thing to ask is: Should we be learning about what to do while doing such projects, or should we be acquiring the expertise before implementation, given that these are very precious forest fragments?"
• Additional reporting by Audrey Tan