LAST Tuesday's crash was at least the second in just over a month in which a container ship hit a major road bridge, raising questions about the safety standards of increasingly large ships and the ability of bridges around the world to withstand crashes.

On February 22 in Guangzhou, a port in southern China, a much smaller vessel carrying stacks of containers hit the base of a two-lane bridge, causing vehicles to fall. Officials said that five people were killed.

The crashes have also raised questions about whether more ships should be required to be ready to drop anchors quickly during port emergencies, and whether tugboats should accompany more vessels as they enter and leave harbours.

There has not been a final report on the Guangzhou incident, and investigators have barely begun to look at what happened in Baltimore. It has been reported that the investigation into the Baltimore disaster could take up to two years.

Ship collision barriers are standard around the support piers of bridges over major waterways like the entrance to Baltimore's harbour, report The New York Times.

The Verrazzano-Narrows Bridge in New York City, for example, has massive barriers of concrete and rocks around the bases of the piers that support it.

It was not immediately clear how old the barriers are around the piers that supported the bridge in Baltimore. The bridge was built almost half a century ago and designed before then. Vessels have become considerably larger in that time.

The crash in Guangzhou occurred on a less important waterway, a minor channel of the Pearl River. The bridge there was being fitted with devices designed to protect the piers in case of any ship crash. The work was supposed to have been completed by 2022 but had been delayed, and the latest target for completion was August of this year, according to China Central Television, the state broadcaster.

Harbour pilots and crews of many large ships have two anchors ready to drop as they enter or leave a harbour, in case an emergency such as a loss of power means that they need to try to stop quickly.

Basil M Karatzas, the chief executive of Karatzas Marine Advisors, a ship inspection company in New York, said that while he had seen tanker crews commonly take this precaution, it was less common for container ships.

Large ships are often accompanied by tugboats as they leave or enter harbours so that the tugboats can push them away from harm if the ship has difficulties.

The ship in Baltimore was exiting the harbour as a spring tide was rushing out of the harbour. The moon was still almost completely full, having reached its fullest less than 24 hours earlier.

Full moons in springtime are associated with some of the largest tidal changes in local sea level. And while Baltimore's harbour experiences fairly small changes even during springtime full moon tides, tidal movements of water could have been a factor in the bridge impact.

"The ebbing tide increases the speed of the water seaward, which effectively has a cumulative effect on the speed of an outbound vessel, and any currents in the water could also have complicated navigation," Mr. Karatzas said.