Graham Farmer tunnel signage has been upgraded after a vehicle sideswiped a broken-down car, injuring the driver.
A report into the Christmas Eve accident, obtained by _The West Australian _ through Freedom of Information laws, indicates that it could have been avoided if the passing motorist had obeyed lane-closed signs.
As a result of the accident, tunnel managers recommended steps be considered to encourage better compliance with signs.
With no emergency lane in the tunnel, the report says the first car stopped in the left-hand lane about 7pm, with its hazard lights flashing. Tunnel managers immediately closed the lane.
But the driver of the second vehicle continued in the closed lane and hit the broken-down car.
The driver of the first car injured an arm.
A Main Roads spokesman said a new signage system was commissioned late last month, allowing written messages to be displayed to motorists at 13 locations on each side of the tunnel. Tunnel managers used to be restricted to a "turn on radio" sign where Traffic Operation Centre staff conveyed a message through a radio broadcast system.
In 2013-14, after the emergency lane was removed and the tunnel converted to three lanes each way, 23 accidents were recorded.
The Main Roads spokesman said there was no noticeable increase in vehicles hitting walls.
FoI documents reveal most tunnel accidents were nose-to-tail incidents. But on April 7 this year, a car clipped the front of another vehicle while changing lanes and hit the wall. No one was injured.
On August 18 last year, a car hit a truck, then the wall. A car sideswiped the wall on April 17 this year.
Over the same period, the incident response vehicle - which can push stranded cars out of the tunnel - attended 229 incidents. These included 47 pushes and 49 debris pick-ups.
The daily average traffic count for 2013-14 was 98,660 vehicles, with a record 120,909 set on March 21 this year.