St John Ambulance says other drivers not getting out of the way is one of the reasons for slow response times to urgent callouts.
A report shows St John has consistently failed to reach its target callout times in urban areas since 2007.
The targets require ambulance officers to attend at least 50% of its urban priority one callouts within eight minutes.
At present, St John meets this target only 46% of the time.
Chief executive Jaimes Wood says poor road etiquette is partly to blame. He says people simply don't get out of the way for ambulances.
However, response times to rural and remote areas have improved with most targets now being met.
But Mr Wood says ultimately St John remains underfunded, and is struggling to manage the increasing demand for its service.