Taper Lengths in TTM: The AGTTM Shift from Old Standards

Introduction

Tapers guide drivers through safe path transitions in and around work zones. Under AGTTM, taper design is not arbitrary — it's calculated, deliberate, and often shorter than what was previously required by the VicRoads Code of Practice.

This evolution in taper length philosophy prioritises a risk-based, practical approach that reflects real-world conditions.

The Taper Length Shift: AGTTM vs VicRoads CoP

Under the VicRoads Code of Practice (now superseded), taper lengths were generally longer, based on conservative assumptions for all road environments. AGTTM introduces a more nuanced model, with:

• Speed-dependent formulas

• Taper types specific to traffic behaviours

• More compact lengths where safe and appropriate

This shift can result in significantly shorter tapers in common settings, especially for lateral shifts and urban closures.

Reference: AGTTM03-21, Table 5.7 and 5.8

Types of Tapers Defined by AGTTM

• Merge taper: For lane closures with merging vehicles

• Lateral shift taper: Small side shifts in alignment

• Diverge taper: For splitting traffic around obstructions

• Opposing tapers: Used in contraflow or shuttle setups

• Chicanes: Designed for speed control

Reference: AGTTM03-21, Section 5.9.1 and Figures 5.11 to 5.21

How to Calculate AGTTM Taper Lengths

• Merge/Diverge Tapers:

• Lateral Shift Tapers:

Where:

•  = taper length (metres)

•  = lane width (metres)

•  = posted speed (km/h)

Compared to the fixed, longer tapers from VicRoads CoP, these AGTTM formulas often reduce site footprint while maintaining intent.

Why Shorter Can Still Be Safer

AGTTM's taper lengths may be shorter, but they are:

• Backed by national consultation and empirical data

• Supported by risk assessment and monitoring requirements

• Designed to balance safety, practicality, and clarity

Common Errors When Transitioning from Old Standards

• Applying VicRoads taper lengths by habit, not need

• Failing to update TMPs or layout libraries

• Using old spacing charts with new taper lengths

Real-World Tip

Use AGTTM calculations as your new baseline. If you wish to extend a taper for added caution, justify it in the TMP. But don’t default back to VicRoads lengths without good reason — especially where site constraints are tight.

What Auditors Now Expect

• Tapers that match AGTTM calculations

• Documented variations (either longer or shorter)

• Clear cone and sign spacing per updated tables

Reference: AGTTM03-21, Table 5.8 and Section 5.10

Final Word

The shift to AGTTM taper lengths reflects modern thinking: fit-for-purpose, risk-based, and efficient.

Shorter doesn't mean less safe. It means smarter planning. Your taper still needs to communicate clearly — and under AGTTM, that message is sharper than ever.