Chicanes and monitoring cameras are being installed on the Ruapehu Rd and Mangateitei Rd rail overbridges between Ohakune and Rangataua.
They were needed because both timber bridges had failing components and could collapse under heavy vehicles, Ruapehu District Council land transport team leader Andrea Nicol said.
The council regularly receives reports of overweight trucks using the two bridges, despite clear signage, other communications on their restrictions and alternative routes being available.
The bridges cross the main trunk line, and a collapse would put the vehicle driver and others at risk.
AdvertisementAdvertise with NZME.
The Ruapehu Rd rail overbridge had a weight limit of 3.5 tonnes, which meant it should only be used by cars or similar-sized vehicles, Nicol said.
To prevent trucks using it, the council is installing chicanes that will physically limit the length of vehicle able to cross it.
The nearby Mangateitei Rd rail overbridge has a higher 70 per cent weight restriction and is also regularly used by overweight vehicles.
Bridge engineers are in the process of determining whether 70 per cent is too high and if it should be made lower.
The installation of compliance cameras at both bridges highlighted the serious nature of the risk, Nicol said. A bridge failure could be catastrophic, and police said people who ignored the weight restrictions could face prosecution.
Heavy vehicles would also speed the deterioration of the bridges, which could lead to them having to be closed.
"We urge all drivers to observe the weight and speed restrictions on bridges and keep themselves, other road users and railway workers safe," Nicol said.