Traffic Counts for Work Zone Planning: Volumes, Peaks and Heavy Vehicles

Traffic counts for work zone planning tell the designer how many vehicles will arrive at the works, in which direction, at what times and how many of them are heavy vehicles. Those numbers decide whether a closure will queue, which hours are workable and where warning signs must go. A two-way daily average is not enough.

Most traffic management problems that look like "bad luck" on the day trace back to a number in the plan that did not reflect the real road: a count taken on a quiet Monday in school holidays, a station 10 km away, or a daily figure used as if it were the peak. This guide explains what data a work zone plan needs, where to find it, how to do a site count properly and how to record it. It is general information. The TMP and TGS must be prepared by suitably qualified people using data the road authority accepts.

What a work zone plan needs from traffic data

Not every job needs every row. A short shoulder job on a quiet street needs far less than a peak-hour lane closure on an arterial road. But the designer should be able to say which data was used and why it is enough.

Reading the numbers: AADT, peaks and direction

AADT (annual average daily traffic) is the total vehicles passing a point over a year divided by the days in the year. It is the figure most often published, and the one most often misused.

• It is two-way unless stated. A two-lane road with an AADT of 8,000 carries about 4,000 vehicles per direction on an average day, but not evenly.
• It hides the peak. A large share of the day's traffic can arrive in a couple of hours.
• It hides direction. In the morning, traffic towards a city centre, port or industrial area can be far heavier than the other way. The queue at one end of a stop/slow site can be twice the other.
• It hides the day. Weekday, Saturday, Sunday, school holidays and long weekends can look completely different.

A worked conversion

Where only an AADT is available, a common rough rule treats the peak hour as about 10% of the daily two-way volume. Queensland's supplement to the national guide uses this as a last-resort fallback for estimating a five-minute count. Using illustrative numbers:

• AADT 8,000 two-way, 8% heavy vehicles
• Peak hour about 800 vehicles two-way
• If split evenly, 400 per direction; if the peak is 60/40, about 480 in the peak direction
• Five-minute equivalent in the peak direction: 480 ÷ 12 = 40 vehicles, of which about 3 or 4 are heavy

Notice how much the direction split changes the answer: 33 vehicles versus 40 in five minutes. That flows straight through to queue length. Our queue length and delay estimator shows how quickly the difference grows.

Where to find traffic counts

Road authority data

• NSW: Transport for NSW publishes count data through its Traffic Volume Viewer, with roughly 600 permanent stations collecting continuously. Some are counter stations that record total vehicles, and some are classifier stations that separate light and heavy vehicles. Hourly data can be viewed and downloaded. Transport for NSW has noted data quality and sensor availability issues with this service, so check the record for gaps.
• Queensland: Transport and Main Roads publishes traffic census data, including AADT for state-controlled roads, through the Queensland open data portal.
• Other states: most road authorities publish some count data or provide it on request. Ask the road manager what is available and what they will accept.

Councils

Councils often hold tube counts for local roads, collected for traffic calming, development applications or safety reviews. Ask the council's traffic team. Our guide to traffic impact assessments versus TMPs explains another place where recent counts may already exist.

Commissioned counts

For larger or longer jobs, commission a count at the site. Common methods include pneumatic tube counters laid across the road, which can record volume, classification and speed over several days; video counts, useful for intersections, turning movements, pedestrians and cyclists; and radar or other roadside sensors. Survey specialists can advise on duration and timing.

Heavy vehicles: why they need their own count

A single number for "vehicles" hides the vehicles that matter most for work zones. Heavy vehicles:

• take up much more queue length than cars
• accelerate slowly from a stop, lengthening stop/slow clearance and reducing discharge rates
• need more distance to stop, especially on downgrades
• need room in tapers, lane shifts and detours, and some cannot use a detour at all
• cause more severe outcomes if they enter a work area

Classified counts in Australia usually follow the Austroads vehicle classification system, which groups vehicles by length and axle configuration from short vehicles through rigid trucks to articulated vehicles and road trains. When using a dataset, check how it defines "heavy".

Where long vehicles dominate, average assumptions break down. Queensland's supplement tells designers to use an average vehicle length that matches the actual traffic mix on outback, mining and towing routes, and the Northern Territory adds a road train allowance to its queue method. On freight routes, check heavy vehicle access arrangements with the road manager and the National Heavy Vehicle Regulator. Our guide to heavy vehicle roadworks detours covers what happens when trucks cannot follow the light-vehicle route.

Doing a site count properly

A short count at the site, at the time the works will run, is often the most relevant figure available. Queensland's supplement, for example, puts an actual five-minute count at the controlled approach during the peak time the site will be occupied first in its order of preference, ahead of estimates from AADT.

How to do it well:

1. Pick the right time. Count at the busiest period the works will run, on a representative day. A Tuesday count does not represent Saturday works.
2. Pick a safe position. Stand off the road, behind a barrier or kerb where possible, with a clear view of the approach. Wear high-visibility clothing.
3. Count each direction separately. The queue at each control point depends only on traffic approaching it.
4. Separate light and heavy vehicles. Note buses, cyclists and oversize vehicles too.
5. Count more than once. Five minutes is a snapshot. Several samples across the window show whether traffic is steady or peaky.
6. Note conditions. Date, time, weather, school term, events, nearby roadworks or incidents.
7. Record it in the plan. Who counted, where, when and the raw numbers.

Site count record

Common counting mistakes

• Using two-way AADT as if it were one direction. This can halve or double the estimated queue.
• Counting at the wrong time. Off-peak counts for peak works, or term-time counts for holiday works.
• Ignoring growth and change. Old data on a road where a new estate, school or warehouse has opened.
• Missing generators. Schools, shopping centres, hospitals, sporting grounds, ports and quarries create sharp local peaks.
• Forgetting the other works. A closure on a parallel road can push traffic onto yours.
• No record. If the count is not written down, nobody can check the assumption later.

A good brief to the designer includes the data available and its source; our traffic management designer brief guide covers what else to include. National guidance on traffic studies is published by Austroads, and permitted work hours set by authorities, discussed in our guide to roadworks hours restrictions, are usually based on the same kind of data.

Next steps

Before your next plan is drawn, confirm the hourly, directional and heavy vehicle numbers it relies on, and where they came from. If you run a traffic control company, TrafWise keeps job details, site notes and shift records together so the data behind each plan stays with the job.