Temporary Speed Limits in TTM: When, Where, and How to Apply Them

Introduction

Temporary speed limits are a critical safety measure — but they're also one of the most misunderstood elements in traffic management.

Under the Austroads Guide to Temporary Traffic Management (AGTTM), reduced speed zones must be risk-justified, consistently applied, and clearly removed when no longer needed.

Let’s break down the when, where, and how.

When to Apply a Reduced Speed Limit

Reduced limits should be applied when:

• Worksite conditions increase risk (e.g. narrow lanes, short taper lengths, no barrier separation)

• Sight distance is reduced

• Workers are close to live traffic

• The safe operating space has been compromised

Reduced speed limits are not to be used to "slow traffic generally" — they must link directly to identified risks.

Reference: AGTTM03-21, Section 5.5.1; QGTTM Part 3, Section 4.5

Posted Speed vs Temporary Speed: Which Do You Use?

This is a common point of confusion.

• The posted speed is the permanent regulatory speed limit for the road

• The temporary (reduced) speed is the risk-based speed limit applied during works

When calculating:

• Sign spacing

• Advance warning distances

• Taper lengths

👉 Use the posted (permanent) speed unless AGTTM or a risk assessment directs otherwise.

The reduced limit is what road users are instructed to follow, but the posted limit defines how far in advance they need to be warned.

How to Set the Right Temporary Speed

Consider:

• Stopping sight distance (SSD)

• Type of work and proximity to traffic

• Type of road user (pedestrian? motorcyclist? cyclist?)

• Visibility and surface condition

AGTTM provides Table 5.6 and Table 5.5 to help guide selection and setup.

What Is the 85th Percentile and How Does It Apply?

The 85th percentile is the speed at or below which 85% of drivers are observed to travel under free-flow conditions.

It’s used in:

• Setting speed zones

• Evaluating if road users are likely to comply

• Determining whether posted or reduced speeds are reasonable

If you reduce the limit too far below the 85th percentile, non-compliance increases and so does risk.

Use the 85th percentile to support:

• Traffic modelling

• Speed selection justification in your TMP

• VMS or enforcement placement decisions

Reference: AGTTM03-21, Clause 5.5.2 and 5.5.3

Real-World Tip

If your reduced speed is 40 km/h but vehicles are still travelling at 70, you need to:

• Review taper length

• Increase warning signage

• Consider enforcement or VMS with speed-activated displays

Don’t just post a sign — engineer the outcome.

Final Word

Temporary speed limits aren’t about slowing down drivers — they’re about matching speed to risk.

Use posted speed for layout design. Use reduced speed only where justified. And use the 85th percentile to challenge assumptions.

Every number on a sign should tell a risk story.