Frustrated ambulance drivers racing to 111 callouts are pleading with Christchurch motorists to pull over and let them pass.
Motorists are ignoring flashing lights and sirens, particularly where there are road works around, St John says.
This is causing delays for ambulances getting to callouts, which could be life threatening.
St John has taken out a radio advertising campaign, asking motorists to "stop and pull over".
St John spokeswoman Christine Prince said motorists appeared confused around roadworks when an ambulance was coming up behind them.
Some motorists were pulling over and stopping in the middle of the road - which often blocked an ambulance's path.
It was "frustrating" ambulance paramedics, she said.
"Motorists are displaying a lack of uncertainty about what to do when there are roadworks and they are not noticing when an ambulance is behind them. Ambulance drivers are battling with motorists. When they don't pull over or when they stop in the middle of the road it creates a hazard which frustrates paramedics.
"The roadworks have created more confusion than usual in the streets of Christchurch. If it is consistently happening it will delay our response time which is a matter of life and death in some cases.
"We are urging motorists to broaden their peripheral awareness," she said.
The Fire Service and police don't seem to have as many problems.
Christchurch deputy chief fire officer Greg Crawford said in the past there were issues with boy racers not pulling over on purpose to hold up fire engines.
But that had now stopped.
"Like other emergency services we are responding to a life or death situation.
"In the past, particularly boy racers, some idiot will think he is smart and won't move out of the way.
"But giving their registration plate number to police sorts them out.
"It doesn't happen again,'' he said.