Brisbane bus drivers are being abused over a road rule that some motorists may not even know exists - even though the law is spelled out in front of them.

When driving in a built-up area with a speed limit of 70km/h or less, drivers must give way to a bus displaying a "give way to buses" sign and indicating to enter traffic from a bus zone, bus stop, bus stop bay, road shoulder or left side of the road.

Give way signs are plastered on the back of Brisbane's buses. Credit:Glenn Hunt

Failing to give way to a bus can attract a maximum fine of $2438.

A Transport and Main Roads spokeswoman said that like other vehicles, buses were required to indicate for five seconds before moving off from being parked.