This detour delay calculator compares the normal route with the detour for the same trip. It shows the extra time and distance for each vehicle, then multiplies that across a day and across the whole closure, so you can put a number on the impact before the road is shut.
Councils, road authorities, residents and clients all ask the same questions about a road closure: how much longer will it take, how far out of the way is it, and is there a better time to do it? A clear answer, with the working shown, gets plans approved faster and cuts complaints.
How it's worked out
Each route gets a travel time from its length and average speed:
- Trip time (minutes) = distance in km ÷ average speed in km/h × 60
- Extra time per vehicle = detour time − normal time
- Extra distance per vehicle = detour km − normal km
- Vehicle-hours per day = vehicles per day × extra minutes ÷ 60
- Extra km per day = vehicles per day × extra km
- Over the closure = the daily figure × the number of days
If you enter a value of time in dollars per vehicle-hour, it also shows the time cost per day = vehicle-hours per day × your figure, and the same over the closure. The box is blank by default. It must be your own figure, your client's or the road authority's, because economic values of time differ by agency, vehicle type and trip purpose.
If the detour comes out no slower than the normal route, the calculator tells you, and reminds you to check that the speeds include intersections and signals.
Detour delay calculator: a worked example
A council is closing a 1.2 km section of a local road for 5 days to replace a culvert. About 4,000 vehicles a day use it. The signed detour is 3.4 km through a residential area.
| Item | Normal route | Detour |
|---|---|---|
| Distance | 1.2 km | 3.4 km |
| Average speed | 50 km/h | 40 km/h |
| Trip time | 1.44 min | 5.1 min |
- Extra time per vehicle: 5.1 − 1.44 = about 3.7 minutes.
- Extra distance per vehicle: 3.4 − 1.2 = 2.2 km.
- Vehicle-hours per day: 4,000 × 3.66 ÷ 60 = 244.
- Extra kilometres per day: 4,000 × 2.2 = 8,800 km.
- Over 5 days: about 1,220 vehicle-hours and 44,000 extra kilometres.
Those numbers make the case for a few changes. Running the work over 3 days instead of 5 would save about 488 vehicle-hours. Opening one lane under stop/slow during the day might cost less delay than a full closure, which you can test with the stop/slow cycle calculator and the queue length calculator.
Getting the inputs right
The answer is only as good as the speeds. The most common mistake is using the speed limit for the detour.
- Use average speed, not the limit. Include stops at intersections, signals, roundabouts, school zones and turns. A suburban detour signed at 50 km/h can average well under that once the stops are counted.
- Drive it at the right time. Drive the detour at the time of day the closure will run, and time it.
- Allow for the extra traffic. The detour roads will carry the closed road's traffic on top of their own, so they may slow down. If they're already busy at peak, the real delay will be higher.
- Count the vehicles. Use counts from the road authority or council, or count on site. A guess at the daily traffic can throw every other number out.
- Think about who's travelling. Buses, trucks and emergency vehicles may need a different, longer route than cars.
Heavy vehicles, buses and pedestrians
A detour that works for cars may not work for everything else. Before you advertise it, check:
- Heavy vehicles. Turning paths at tight intersections, weight limits on bridges, low clearances and residential streets that ban trucks. A separate heavy-vehicle detour is common. Our guide to heavy-vehicle detours around roadworks covers the checks.
- Buses. Talk to the operator early about stops that will be skipped and temporary stops.
- Emergency services. Tell ambulance, fire and police, and keep access for them through the closure if you can.
- Residents and businesses. Driveway access inside the closure, deliveries and parking.
- Pedestrians and cyclists. If the footpath is closed too, the pedestrian detour needs its own plan.
In NSW, TS 05492 s11.6 sets footpath widths for temporary paths: a desirable minimum of 1.8 m for a partly closed footpath, 2 m for a temporary footpath, and no less than 1 m at an isolated unavoidable hazard such as a pole. The accessible pedestrian detour checklist goes through the rest.
Signing the detour
Drivers need to know about the detour before they reach the point where they have to turn. TS 05492 Table 66 sets the minimum distance from a portable variable message sign (VMS) to the next decision point, such as the turn onto the detour:
| Speed | Minimum distance from the VMS to the decision point |
|---|---|
| 40 km/h | 65 m |
| 50 km/h | 90 m |
| 60 km/h | 115 m |
| 70 km/h | 140 m |
| 80 km/h | 170 m |
| 90 km/h | 200 m |
| 100 km/h | 235 m |
Decision points include intersections, merges, exits, signals, crossings, sharp curves and speed changes. Detour signs at every turn, and a sign confirming the end of the detour, stop drivers getting lost on local streets. Our guide to road closure and detour plans covers the full layout, and the sign layout builder works out advance sign positions.
State differences
The arithmetic is the same everywhere. What changes between states is the approval process and the signing rules:
- NSW: TS 05492 (June 2026), which replaced TCAWS v6.1, sets signing and VMS rules. Closures on state roads usually need a road occupancy licence; local roads go through the council.
- Queensland: the Austroads Guide to Temporary Traffic Management (AGTTM) with the differences in QGTTM Part 3. Work on state-controlled roads needs TMR's permission.
- Victoria, WA and SA: AGTTM with each state's code of practice and permit system.
Whatever the state, the approved traffic management plan and TGS decide the detour route and signs.
When to get professional advice
Use the calculator to compare options and explain the impact. Get a qualified traffic management designer to prepare the plan when:
- the closed road is a state road, a bus route or a freight route
- the detour adds more than a few minutes for thousands of vehicles a day
- the detour uses roads that are already near capacity at peak
- heavy vehicles or oversize loads need their own route
- the closure needs a permit or road occupancy licence
A good detour plan, with the delay worked out and the heavy-vehicle route checked, is usually what gets a closure approved first time.