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Stops and goes of traffic signals

This booklet describes how the safety and/or effectiveness of traffic signals could be improved. It draws attention to those elements of traffic signals that auditors have frequently found to compromise safety and/or efficiency.

5. Intersection and lane layout

5.1 Opposed right-turn lanes

Exclusive right-turn lanes should be provided whenever possible. Opposed right-turn lanes should line up, and not be offset, allowing right turners waiting for a filter turn as much forward visibility past queued opposing right turners as possible.

Many 14 m carriageways had their two-lane shared approaches reconfigured to include an exclusive right-turn lane, without relocating the centre line. This resulted in the opposed right-turn lanes being offset.

Safety and efficiency issues

Poorly aligned right-turn lanes can lead to:

poor intervisibility between right turners and opposing through traffic, resulting in a higher right-turn-against crash rate
drivers concentrating too much on opposing through traffic, overlooking pedestrians or cyclists to whom they must also give way
some drivers being hesitant, reducing intersection capacity
increased inter-green time due to the longer tracking paths.

Recommended treatments

Ensure right-turn bays line up (ie, ‘back to back’ design).
Reducing the right-turn lane width minimises the offset between opposed right turners, further increasing forward visibility.
Where opposed right-turn lanes are not possible (eg, due to the inability to ban kerbside parking or tracking paths of vehicles), consider a different phasing operation (eg, split approaches) or a right-turn ban for one direction.

Figure 1 Offset right-turn bays (and hidden primary lantern)

Figure 2 Right-turn bays incorporated into solid median achieving excellent forward visibility

5.2 Captive turn lanes

Captive turn lanes are created when a mid-block through lane leads directly into a turn lane at an intersection. It is important that advanced warning is given to motorists that they may only turn from these lanes.

Ideally, drivers should be channelled into through lanes at intersections (or shared through and turn lanes) and should not be confronted with captive turn lanes.

Captive turn lanes with insufficient warning were found quite frequently during the signal audits and appear to result from the engineering plans not showing how to tie the intersection lane layout into the existing mid-block markings.

Safety and efficiency issues

Captive turn lanes can lead to:

erratic and undesirable driver behaviour, including sudden lane changes or through movements when only turning movements are allowed, potentially resulting in a higher crash rate
drivers unnecessarily slowing or stopping close to the intersection for a desired lane change, impacting on the intersection capacity.

Recommended treatments

Channel drivers into through lanes whenever possible (ensure that engineering plans show the tie-in into the mid-block layout).
Where captive lanes cannot be avoided (eg, where two approach lanes in the mid-block lead into the stem of a T intersection with exclusive turning lanes), ensure that drivers have sufficient prewarning by lane arrows and possibly signs.

Figure 3 Captive left-turn lane with insufficient warning to motorists

Figure 4 Lane markings advising motorists of captive turn lanes

5.3 Slip lanes

Slip lanes help to make intersections more compact, and enable traffic signal posts and lanterns to be placed closer to drivers' line of sight. Decision-making processes for motorists are generally simplified, resulting in a safer intersection layout. While some pedestrians voice reservations about slip lanes, they do remove the conflict that occurs when left turners and parallel pedestrians proceed together.

Slip lanes should comply in layout with guidance given in Austroads Part 5 (1988). For sign posting details and provisions for visually impaired pedestrians, refer to RTS 9, 1993 and RTS 14, 2003, respectively.

Safety and efficiency issues

Poorly designed slip lanes can lead to:

poor intervisibility between pedestrians and left turners, if the crossing position is too far around the corner
drivers having problems observing traffic to give way to if the slip lanes are not of the high-entry-angle type or if the kerb radii are too large (because some people have problems turning their heads and door pillars may obstruct visibility). Driver hesitance and increased crash rate are possible consequences
increased turning speeds, resulting in loss of control or failure to give way
not enough room for pedestrians (eg, during the school peak) or street furniture, if islands are undersized.

Recommended treatments

Slip lanes should be of appropriate size and of a high-entry-angle type, with adequate corner radii.
The location of the pedestrian crossing point should provide sufficient intervisibility.
Pedestrian priority issues can be addressed using a signalised slip lane crossing or a priority (zebra) crossing (possibly on a raised platform).