10.1 Dimensions, Rear Overhang and Projecting Loads
Key Takeaways
Ordinary heavy vehicles generally use 2.5 m width and 4.3 m height; qualifying Safer Freight Vehicles can use 2.55 m width.
Eligible prime-mover/semi-trailer and rigid-truck/trailer combinations can be up to 20 m under the current rules, subject to geometry and equipment requirements.
For the general rear-projection warning rule, use a daytime flag at least 300 mm square or a night red light visible at least 200 m; oversize exemptions have separate warnings.
Measure the loaded travel configuration
A vehicle’s dimensions include its load unless the governing rule expressly excludes an item. Know the actual length, width, height and overhang after loading, with equipment secured for travel. Choose a vehicle and load position that minimise dimensions. A driver should not solve a poor loading choice by assuming an oversize sign makes any projection lawful.
The current Heavy Vehicle (Mass, Dimension and Loading) National Regulation and NHVR guidance govern category details. The July 2026 handbook is useful, but some simplified tables and prose differ. Use the current category and conditions rather than apply one number indiscriminately to all trucks, buses and trailers.
Common limits and exceptions
| Dimension or vehicle | General reference | Conditions to remember |
|---|---|---|
| Width | 2.5 m | Defined exclusions and special categories apply |
| Qualifying Safer Freight Vehicle | Up to 2.55 m | Must meet required safety and certification criteria; not every trailer qualifies |
| Height | 4.3 m | Specific double-deck bus, livestock and vehicle-carrier provisions differ |
| Ordinary rigid truck length | 12.5 m | Includes load; bus categories have separate provisions |
| Rigid bus | Up to 14.5 m under its category | Access authorisation needed where applicable above 12.5 m |
| Articulated bus | 18 m | Subject to its category and access requirements |
| Eligible truck/trailer or prime-mover/semi-trailer combination | Up to 20 m | August 2026 geometry and equipment conditions apply |
| B-double | Separate provisions, potentially up to 26 m | Configuration and route-authorisation conditions apply |
A 4.4 m double-deck bus or a qualifying 4.6 m livestock or multi-deck vehicle carrier is not evidence that every truck may be 4.6 m high. The category, notice and network matter. The August 2026 rules also allow a qualifying specified semi-trailer to be up to 4.6 m: it must be non-PBS, have air suspension with effective dampers, and have deck height no more than 1.2 m over at least half its deck length. In the permitted single-semi, B-double or B-triple combination, actual mass must not exceed 90% of its prescribed combination mass requirement. Route access still needs checking. A high-cube container alone does not establish these conditions. Every vehicle must also respect the actual road’s clearance signs.
The 20 m reform
From 1 August 2026, specified prime-mover/semi-trailer and rigid-truck/pig, dog or tag-trailer combinations can exceed 19 m up to 20 m if they meet the new conditions. These include matching dimensions and equipment. Trailers above 10 t Gross Trailer Mass in a combination longer than 19 m and not longer than 20 m need the specified ADR 38/05-or-later braking and towing-vehicle power arrangements.
Do not convert that reform into unconditional general access for every 20 m configuration. B-doubles retain their own requirements. A combination that does not meet the eligibility conditions needs the appropriate alternative authority or configuration.
Rear overhang
Rear overhang is measured from the legally defined rear-overhang line to the rearmost point of the vehicle or load. For one rear axle, the line is its centre. For equal-tyred rear groups, it is generally the group centre. Unequal tyre arrangements and steerable axles can change the location; use the proper definition.
For an ordinary rigid truck, the general maximum is the lesser of 3.7 m or 60% of wheelbase. With wheelbase 5 m, 60% is 3 m, so the limit is 3 m. With wheelbase 7 m, 60% is 4.2 m, so 3.7 m governs. The smaller limit is selected; the percentages are not added to the fixed distance.
For a semi-trailer, dog trailer or tag trailer, the ordinary maximum is the lesser of 3.7 m or 60% of the distance from the trailer’s front articulation point to its rear-overhang line. For a pig trailer, compare 3.7 m with the distance from the front of its body or load-carrying area to that line and use the smaller. In the particular eligible 19–20 m semi-trailer combination, the maximum becomes the lesser of 4 m or 60% of the articulation-to-overhang-line distance, with the specified extended geometry and prime-mover wheelbase table also satisfied. A combination’s length alone does not qualify it. Certain multi-deck vehicle carriers have a separate 4.9 m provision when a carried vehicle overhangs the rear.
Front, side and rear projections
The general front load projection limit is 1.2 m in front of the motor vehicle’s headlights. The general side projection limit is 150 mm beyond the outer extremity on either side of the relevant vehicle. These allowances do not permit exceeding the total lawful width, length or rear overhang. A full-width 2.5 m truck cannot add 150 mm cargo on each side and claim ordinary 2.8 m width.
The general rear warning rule applies when the load projects more than 1.2 m behind the relevant vehicle, projects from a pole-type trailer, or is not readily visible to a person following immediately behind. During daytime, fix a brightly coloured red, red-and-yellow or yellow flag at least 300 mm by 300 mm to the extreme rear. At night, use a clear red rear-facing light visible at least 200 m. A flag does not replace the night light.
Oversize exemption conditions have separate requirements, including specified 450 mm flags for certain side projections. Do not confuse those with the general rear-projection flag. Warning equipment alerts others; it does not remove the need for lawful dimensions, appropriate restraint and a safe load without dangerous projections.
Key decisions
Measure all loaded dimensions, identify the vehicle category and apply the exact authority. Check overhang and warning requirements separately, because satisfying one does not prove compliance with the others.
Official sources
Current national MDL Regulation, dimensions and section 37A; NHVR August 2026 changes; NHVR size and projection. Sources checked 10 October 2026.
An ordinary rigid truck has a 5 m wheelbase. What is its general rear-overhang limit under the lesser-of rule?
3.0 m
3.7 m regardless of wheelbase
4.2 m
8.7 m by adding wheelbase and overhang
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