This stopping sight distance calculator shows how far a vehicle travels from the moment a driver sees a hazard to the moment it stops. It splits that distance into the part covered while the driver reacts and the part covered while braking, at the speed, reaction time and grade you enter.
It is a planning and teaching aid. On a real job the sign spacing, controller sight distance and queue warning come from the current standard for your state and the approved traffic guidance scheme (TGS). In NSW that standard is Transport for NSW's TS 05492 Traffic Control at Work Sites, which replaced the old TCAWS manual (v6.1) on 25 June 2026.
How it's worked out
Stopping sight distance (SSD) has two parts: reaction distance and braking distance. The calculator uses the standard road-design form:
SSD = (RT × V) ÷ 3.6 + V² ÷ (254 × (d + 0.01 × a))
- V is the approach speed in km/h.
- RT is the reaction time in seconds (default 2.0).
- d is the coefficient of deceleration (default 0.36).
- a is the grade in percent, positive uphill and negative downhill (default 0).
The first part, (RT × V) ÷ 3.6, is the distance travelled at full speed while the driver sees the hazard, recognises it and gets their foot on the brake. Dividing km/h by 3.6 turns it into metres per second. The second part, V² ÷ (254 × (d + 0.01a)), is the distance to brake to a stop. Because speed is squared, braking distance grows much faster than speed does.
It also shows the speed in metres per second and the time to stop (total distance ÷ metres per second).
Austroads road design guidance uses different reaction times and deceleration values for different cases, such as cars versus trucks and normal versus constrained design. That's why every input is editable. Wet, worn or loose surfaces and heavy vehicles all need longer distances than the defaults give.
Stopping sight distance calculator: common speeds
Here is what the calculator gives with the defaults (2 seconds, 0.36, flat road):
| Speed | Reaction | Braking | Total |
|---|---|---|---|
| 40 km/h | 22 m | 17 m | 40 m |
| 50 km/h | 28 m | 27 m | 55 m |
| 60 km/h | 33 m | 39 m | 73 m |
| 70 km/h | 39 m | 54 m | 93 m |
| 80 km/h | 44 m | 70 m | 114 m |
| 90 km/h | 50 m | 89 m | 139 m |
| 100 km/h | 56 m | 109 m | 165 m |
| 110 km/h | 61 m | 132 m | 193 m |
At low speed most of the distance is reaction. By 100 km/h braking is about two thirds of it. That's the practical lesson for a toolbox talk: a driver who is looking at a phone for even a second has used up a big part of their stopping distance before they touch the brake.
The NSW work-zone figures
TS 05492 Table 63 gives guide values for stopping sight distance on level pavement at work sites:
| Speed | Car | Laden truck |
|---|---|---|
| 50 km/h | 60 m | 80 m |
| 60 km/h | 80 m | 100 m |
| 70 km/h | 100 m | 130 m |
| 80 km/h | 120 m | 160 m |
These sit a little above the calculator's defaults (114 m against 120 m for a car at 80 km/h). A reaction time of 2.5 seconds gives about 126 m at 80 km/h, which shows how sensitive the result is to the inputs. Where the NSW table applies, use the table.
Table 63 also gives a simple grade correction: add 2% to the stopping sight distance for every 1% of downgrade, and take 2% off for every 1% of upgrade. The calculator handles grade through the formula instead, by changing the effective deceleration.
A worked example
A crew is planning a lane closure on a rural road. Drivers approach at 80 km/h down a 6% grade towards the work.
- Enter 80 km/h, leave the reaction time at 2 seconds and the deceleration at 0.36.
- Enter a grade of −6.
- Reaction distance: (2 × 80) ÷ 3.6 = 44 m.
- Braking distance: 80² ÷ (254 × (0.36 − 0.06)) = 6,400 ÷ 76.2 = 84 m.
- Total stopping sight distance: about 128 m, and it takes about 5.8 seconds to stop.
On a flat road the same car needs 114 m, so the downhill adds about 14 m. If the same road climbed at 6% instead, the total would drop to about 104 m. For a laden truck, start from the Table 63 figure of 160 m at 80 km/h. A 6% downgrade at 2% per 1% adds 12%, so plan for about 179 m.
So queue warning and the controller position need extra care on the downhill side. The sight distance check tests a controller position against the 1.5D rule, and the speed and distance converter shows how far a vehicle travels each second.
Stopping distance versus the sight distance rules
Stopping sight distance is physics. The sight distances you must meet on a job are rules, and in NSW most of them are multiples of Dimension D, the speed of the zone drivers are in before the point, read as metres.
- Traffic controller, STOP/SLOW bat or portable traffic control device: at least 1.5 × D (120 m at 80 km/h).
- End of a queue on an open road: 2 × D (160 m at 80 km/h). In urban areas, 1.5 × D.
- Working in gaps in traffic: 3 × D with a lookout, 6 × D without.
Those figures come from TS 05492 Tables 73 and 87. Compare them with the table above and you can see why the rules build in a margin over a car's stopping distance: they also have to cover trucks, inattentive drivers and the time a driver needs to understand what they're looking at. Our guide to stopping sight distance at work zones covers this in more depth, and lane closure taper lengths and sign spacing shows where the numbers end up on a plan.
Queensland and other states
Queensland uses the Austroads Guide to Temporary Traffic Management (AGTTM) with its own differences in QGTTM Part 3. It doesn't use Dimension D the way NSW does. Sign spacing there is 15 m up to 55 km/h, 45 m from 56 to 65 km/h and equal to the speed above that, with a single sign at double the spacing. PREPARE TO STOP distances come from QGTTM Part 3 Table 4.4 and are measured from the predicted end of the queue.
Victoria, WA, SA and the territories also build on AGTTM with their own codes of practice. The stopping distance formula is the same everywhere because it's physics, but the distances you set out on site come from your state's rules and the TGS.
When to get professional advice
Use this calculator to understand and explain stopping distance, to sanity-check a layout or to brief a crew. Get a qualified traffic management designer involved when:
- the road has steep grades, tight curves or crests that limit what drivers can see
- heavy vehicles, buses or B-doubles make up a large share of traffic
- the surface is wet, loose or newly sealed
- the TGS distances can't be achieved on site and the layout needs to change
Any change to the layout beyond the allowed tolerances is a TGS change, and the designer has to approve it. Our guide on how to read a TGS explains where these distances appear on the drawing.