Queue Lengths in TTM: Predicting, Monitoring, and Managing End-of-Queue Hazards

Introduction

In roadwork zones, queue lengths are more than an inconvenience – they’re a leading cause of high-speed crashes. End-of-queue (EOQ) collisions often involve inattentive drivers and occur with little warning.

AGTTM explicitly addresses this risk, requiring designers and planners to forecast, monitor, and mitigate queue build-up. Here's how to do it properly.

Why Queue Lengths Matter

When vehicles are forced to stop or slow unexpectedly:

• Drivers often fail to notice queues in time

• Heavy vehicles have longer braking distances

• Rear-end collisions become severe, especially above 60 km/h

Reference: AGTTM03-21, Section 4.8.4; AGTTM10-21, Section 2.4.3

How to Estimate Queue Lengths

AGTTM provides general guidance but not a fixed formula. Practical methods include:

• Historical data or previous site observations

• Peak hour traffic modelling

• Site inspection for congestion indicators (e.g. nearby intersections, school zones)

As a rule of thumb, any queue over 150 m on roads 70 km/h and above is considered high risk.

Monitoring Queue Build-Up

Queue conditions can change quickly. Effective monitoring includes:

• Visual observations by field staff

• Use of CCTV or radar sensors

• Instructions in TMP for how to report and respond to queues

In Queensland, QGTTM requires specific monitoring triggers to be defined in the TMP.

Reference: QGTTM Part 3, Section 4.8.3

Devices for Queue Protection

End-of-Queue (EOQ) Protection Options:

• Portable Variable Message Signs (VMS): "QUEUED TRAFFIC AHEAD"

• Advance warning signage at extended distances

• Additional tail vehicle with arrow board

• Use of rumble strips (where appropriate)

Reference: AGTTM03-21, Sections 4.8.4 and 5.8

Buffer and Taper Considerations

If your queue estimate suggests significant stacking, AGTTM recommends:

• Extending advance warning area

• Including a longer taper or shoulder refuge zone

• Placing Truck Mounted Attenuators (TMAs) at risk zones

Reference: AGTTM03-21, Section 4.5 and 5.5.3

Real-World Tip

Tail vehicle positioning is critical. It should sit at the back of the queue, not the start of the taper, during high-volume phases.

Also, VMS boards are only effective when placed well before the queue starts – not at the taper entry.

What Auditors Look For

• Was queue length discussed in the TMP?

• Is there EOQ monitoring and response protocol?

• Are devices and buffer zones scaled for the speed zone and traffic volume?

Neglecting queues is one of the top findings in post-incident reviews.

Final Word

Queue management is not about adding more cones. It’s about foreseeing the danger and managing expectations.

Every queue can be predicted, monitored, and managed – and every planner has the tools to do it.