A car heading into Johor Bahru collided with a bus coming into Singapore in the pre-dawn hours on May 24 when traffic flow direction at Woodlands Checkpoint was reversed before barriers were put in place.

The driver of the Singapore-registered car and his three passengers were taken to hospital after the accident, which happened at 5.28am, the Immigration and Checkpoints Authority (ICA) said in a statement.

There were no reported injuries among the bus passengers, and the driver of the bus declined medical assistance.

ICA said: "This accident should not have happened. ICA is treating it very seriously, and we are assisting the Traffic Police, who are investigating the accident."

After the accident, ICA officers immediately rendered help and managed the traffic situation, and also alerted the Singapore Civil Defence Force and police.

ICA said it has been carrying out contra-flow traffic operations during peak periods to manage heavy traffic at the land border crossings.

Contra-flow traffic operations are temporary arrangements in which traffic flows in the opposite direction of the usual flow.

At Woodlands Checkpoint, that includes converting the departure lane for heavy vehicles at the Causeway into an arrival lane, to speed up the arrival of heavy vehicles that might be blocking traffic, said ICA.

During this time, if there is a surge in the car traffic leaving Singapore, part of the departure cargo zone may be converted to move along cars leaving Woodlands Checkpoint, to minimise the tailback of traffic at the BKE.

ICA said: "As part of ICA's safety procedures, barriers would be set up to divert departing car traffic away from the contra-flow lane, with arriving heavy vehicles using the departure lane for heavy vehicles.

"However, in this accident, preliminary investigations found that the contra-flow operation was activated before barriers were put in place, which was not in accordance with our safety procedures."

ICA added that it implemented a safety pause immediately after the accident, and suspended contra-flow operations at both land checkpoints at the weekend.

It also said it will undertake a full review of safety measures before restarting the contra-flow operations. Motorists can expect some delays in the meantime, ICA added.