The Pan-Island Expressway (PIE) is the highway where the most traffic accidents occurred over the past three months.
Singapore's longest highway, at 42.8km, had at least 441 accidents, with the highest concentration around the Eng Neo Avenue exit. There were 48 accidents there, evenly split in both directions.
The numbers were gleaned from more than 2,000 tweets by the Land Transport Authority's @LTAsg Twitter account between May 26 and Aug 22. The automated traffic updates are based on real-time surveillance cameras and the Expressway Monitoring Advisory System.
Although the PIE had the highest number of accidents, it fell behind the Central Expressway (CTE) and Seletar Expressway (SLE) when the number of accidents was set against the length of the road.
The 15.5km CTE had at least 164 accidents, or 10.58 per km, and the 12km SLE had 125 accidents, or 10.42 per km, compared with the PIE's 10.3 per km.
On Aug 9, a horrific CTE accident before the Yio Chu Kang exit left a Singaporean, his Korean girlfriend and her parents dead.
Aside from Eng Neo, other hot spots include a stretch of the PIE near Paya Lebar, the CTE near the PIE (Changi) exit and the SLE near Mandai Road. Each recorded at least 25 accidents.
Transport experts had differing explanations for the accidents on expressways. National University of Singapore transport researcher Lee Der Horng said many crash areas were on the PIE because the oldest expressway was not built to handle the high volume of traffic that now flows through daily.
He said there is also some undulation on parts of the PIE, where the road rises and falls, and affects how far ahead a motorist can see. "The driver may not be able to see traffic in front of him clearly until he moves to the top of the hump, but because of his speed, it might be too late," he said.
That, coupled with heavy and fast-moving traffic, is what makes parts of the PIE near Mount Pleasant and Stevens Road dangerous, he added.
Like the Eng Neo Avenue to Adam Road portion of the PIE, large curves - such as the one at the CTE near the PIE (Changi) exit - could also cause a "shockwave", where high-speed traffic meets slower traffic with disastrous consequences.
Dr Park Byung Joon, head of the urban transport management programme at SIM University, agreed with this assessment, but added that most accidents occur as drivers enter or leave the expressway.
"Accidents are prone to happen where there is a criss-crossing in the flow of cars," he said. "When you change lanes or try to exit, you have to look at the sides and not just in front. If the car in front of you then brakes, your reaction will likely be slower."
He said that accidents can also happen in tunnels, such as the CTE tunnel near Merchant Road, as drivers expect traffic to be fast-moving.
Retired LTA planner Gopinath Menon felt the accident rate reflected heavy use of expressways, and that expressway accidents sometimes result in secondary accidents.
"We have 161km of expressway (out of a 3,400km road network) carrying more than half the traffic volume of the roads," he said.
"Usage is high, traffic is fast and any breakdown compounds the problem. I wouldn't blame road design for the accidents."