Key takeaways
- Stopping sight distance is reaction distance plus braking distance, and it grows with the square of speed, so small speed increases need much more room.
- At a work zone the thing a driver must stop for is often the end of the queue or a traffic controller, not the taper, so sight distance has to be checked to those points.
- Downgrades, wet or loose surfaces, heavy vehicles and drivers who have not slowed to the temporary limit all lengthen the real stopping distance.
- Sign spacing tables and sight distance tables do different jobs; the TGS designer applies both from the current guide, not from a formula on site.
- Crests, curves, parked plant and the signs themselves can hide a hazard, so drive the approach at traffic speed before opening the site.
Stopping sight distance at a work zone is the length of road a driver needs to see a hazard, react and brake to a stop before reaching it. It sets the minimum visibility to traffic controllers, queues, tapers and plant, and it explains why sign spacing grows with speed. Get it wrong and every other device on the TGS is working against physics.
This guide explains the physics in plain terms, works through some illustrative numbers, and shows where sight distance quietly fails on real sites. It is written for supervisors, implementers and designers who want to understand why the tables say what they say. It does not replace the tables, and it is not engineering advice: the traffic guidance scheme (TGS) must be prepared by a suitably qualified person using the current guide and the road authority's requirements.
What stopping sight distance actually measures
Stopping sight distance (SSD) has two parts.
- Reaction distance. The distance travelled between the moment a hazard becomes visible and the moment the driver's foot hits the brake. Road design guidance uses a design reaction time that covers perceiving the hazard, recognising it as a hazard, deciding to brake and moving the foot. Austroads road design guidance commonly works with values around 2 to 2.5 seconds, with the longer value typically used where drivers are less alert, such as long rural roads.
- Braking distance. The distance travelled while decelerating to a stop. It depends on speed, tyre and surface grip (expressed as a deceleration coefficient) and the grade of the road.
The widely used engineering form is:
SSD = (V × t ÷ 3.6) + V² ÷ (254 × (d + 0.01 × G))
where V is speed in km/h, t is reaction time in seconds, d is the deceleration coefficient and G is grade in percent (positive uphill, negative downhill). The first term is reaction distance in metres; the second is braking distance in metres.
The key point is in the second term: braking distance grows with the square of speed. Going from 80 km/h to 100 km/h is a 25% increase in speed but roughly a 56% increase in braking distance.
Worked examples: how quickly the distance grows
The table below uses a 2.5 second reaction time and a deceleration coefficient of 0.36 on level ground. These inputs are typical of desirable values in Australian road design practice, but they are used here only to illustrate the shape of the problem. They are not design values for your site, and they are not the figures in the AGTTM tables.
| Approach speed | Reaction distance | Braking distance | Total (illustrative) |
|---|---|---|---|
| 40 km/h | 28 m | 17 m | about 45 m |
| 60 km/h | 42 m | 39 m | about 81 m |
| 80 km/h | 56 m | 70 m | about 126 m |
| 100 km/h | 69 m | 109 m | about 179 m |
| 110 km/h | 76 m | 132 m | about 209 m |
Two variations show why the inputs matter.
- A 6% downgrade at 100 km/h. The grade term reduces the effective deceleration from 0.36 to 0.30, and the braking distance rises to about 131 m. The total becomes about 201 m, roughly 22 m more than on level ground.
- A shorter reaction time and higher grip. Using 2.0 seconds and 0.46 at 100 km/h gives about 141 m. Lower values like these are sometimes accepted as an absolute minimum in constrained permanent road design. At a work zone, where drivers are dealing with unfamiliar layouts, signs and people on the road, assuming the most favourable driver is rarely sensible.
You can test other speeds and grades with our stopping distance calculator. Treat the output the same way: a planning estimate that helps you ask the right question, not a substitute for the guide.
Why work zones make sight distance harder
Permanent roads are designed so that drivers can see far enough ahead at the design speed. Temporary traffic management breaks that assumption in several ways.
The hazard moves upstream
On a stop/slow site the first thing a driver has to stop for is usually not the work area. It is the traffic controller or portable traffic control device, and once traffic is held, it is the back of the queue. A queue can grow back past a crest or around a bend that the TGS designer never expected to matter. Queensland's supplement to the AGTTM, for example, requires the primary PREPARE TO STOP sign to be positioned from the predicted end of queue, not from the controller, using the sight distance table in the guide. That is SSD logic applied to the real hazard.
Drivers have not slowed down yet
A temporary speed limit only shortens stopping distance if drivers comply. Approach speeds often stay close to the permanent limit for some distance after the first temporary speed sign. Queensland's guidance deals with this by requiring the higher speed to be used for sign spacing in the first 200 m after a speed change. Whatever your state's rule, the lesson is the same: design and check for the speed drivers are likely to be doing, not the speed on the sign.
The road is not in its design condition
Milled surfaces, loose aggregate, fresh seal, water from saw cutting and mud tracked out by trucks all reduce grip. A wet, loose or contaminated surface lowers the deceleration coefficient and lengthens the braking distance. Night works add another problem: drivers may not see a dark-coloured stopped vehicle or a person until their headlights reach it. Our guide to night roadworks lighting and glare covers that side of visibility.
Heavy vehicles behave differently
Trucks generally need more distance to stop than cars, even though the driver's higher eye position lets them see over some crests. Road design guidance treats truck stopping separately. On freight routes, steep descents and sites with a high share of heavy vehicles, raise this with the designer rather than assuming car-based figures cover everyone.
Sign spacing is not the same as stopping sight distance
People often assume the distance between advance warning signs is the stopping distance. It is not. The two do different jobs.
- Sign spacing gives drivers time to read one message, understand it and prepare for the next. In AS 1742.3 and the AGTTM, spacing for higher-speed roads is expressed as a dimension that increases with speed. Several signs together build a warning sequence that starts well before the hazard.
- Sight distance checks that, at each critical point, the driver can actually see the sign, controller, queue or taper from far enough away to respond.
A site can have every sign at the right spacing and still fail on sight distance: for example, a ROADWORK AHEAD sign placed just over a crest is at the right chainage but invisible until the driver is almost on it. Equally, a long clear straight does not remove the need for the sign sequence. Our guide to road work signs and what they mean explains the sequence from the driver's point of view.
Where sight distance fails on real sites
The failures that cause near misses are usually ordinary. Look for these when you review a TGS or set up a site.
- Crests. The controller is placed at the start of the work area, which happens to be just beyond a crest. Drivers see a person with a STOP bat only when they are already close.
- Horizontal curves. Roadside vegetation, cuttings, noise walls and parked vehicles on the inside of the curve block the line of sight to the queue.
- Signs hiding hazards. A large multi-message sign or VMS trailer placed on the inside of a bend can hide the controller behind it.
- Plant and stockpiles. A truck waiting to tip, a stockpile or a stack of barriers sits in exactly the spot that gave drivers their view.
- Sun and glare. A low sun directly behind the controller, early morning or late afternoon, can make a person almost invisible.
- Queue growth. The site worked at 9 am but by 3 pm the school traffic has pushed the queue back over the crest.
- Side roads and driveways. Drivers joining between the warning signs and the hazard have not seen the sequence and may have much less distance to react.
How to check sight distance on site
Sight distance is not something to approve from the office alone. A practical approach for supervisors and implementers:
| Check | How to do it | What to record |
|---|---|---|
| Drive the approach | At traffic speed, in a vehicle, from beyond the first sign | Where each device first becomes clearly visible |
| Controller visibility | Observer stands at the control point in PPE | Distance at which they are clearly visible to drivers |
| End-of-queue position | Mark the predicted end of queue (a cone or bollard on the shoulder is common practice) | Whether that point is visible over crests and around curves |
| Grade and surface | Note downgrades and surface condition on the approach | Wet, loose or milled surfaces on the approach |
| Obstructions | Look for vegetation, parked vehicles, plant, signs and stockpiles | Anything blocking the line of sight |
| Light conditions | Check at the actual shift time, including sun angle | Glare or darkness issues |
| Side roads | Drive in from each side road and driveway | Warning available to joining traffic |
If the check fails, do not improvise a new layout. Stop, protect the crew and refer it to the supervisor and the designer. Common fixes, such as moving the control point back before a crest, extending the warning sequence, adding a repeater sign or a second controller to slow traffic, or reducing speed, all need to be assessed and authorised. Our guide to how to read a TGS explains which elements the crew can and cannot change. The national guidance is published by Austroads, and in Queensland the supplement and MUTCD Part 3 are published by Transport and Main Roads.
Keep checking through the shift
Sight distance changes as the day changes. Re-check it when:
- traffic volume rises and the queue grows towards a crest or bend
- the sun moves into drivers' eyes
- rain starts, or the work makes the surface wet or loose
- plant, deliveries or stockpiles are moved
- the work area or control point moves along the road
- lighting conditions change at dusk
Record the checks in the inspection record with the time and what you saw. A note that says "sight distance to queue lost over crest at 3:10 pm, queue end marker moved and supervisor advised" is far more useful than a tick in a box. For the speed side of the equation, see our guide to work zone speed limits in NSW.
Next steps
Before your next job on a high-speed or hilly road, drive the approach with the TGS in hand and ask where a driver would first see the controller and the end of the queue. If you run a traffic control company, TrafWise keeps the issued TGS, inspection records and shift notes together so the crew and the office are working from the same information.
Frequently asked questions
What is stopping sight distance at roadworks?
It is the distance a driver needs to see a hazard, react and brake to a stop before reaching it. At roadworks the hazard may be a traffic controller, a stopped queue, a taper or plant. The required distance depends on approach speed, reaction time, road grade and surface grip, and is taken from the guide adopted for the project.
How do you calculate stopping distance?
A common engineering form is reaction distance plus braking distance: speed in km/h times reaction time divided by 3.6, plus speed squared divided by 254 times the deceleration coefficient adjusted for grade. Results are illustrative only. Design values come from the current Austroads guidance and state supplements, applied by a suitably qualified person.
Why is a stopped queue more dangerous than the work area?
Drivers expect the work area because the signs describe it, but a queue can form well before the first sign drivers associate with stopping. If the end of the queue sits over a crest or around a curve, approaching drivers may not see stationary vehicles in time. That is why end-of-queue warning and monitoring matter.
Does a lower temporary speed limit reduce the sight distance needed?
Only if drivers actually slow down. Many drivers are still travelling close to the permanent speed when they pass the first temporary speed sign, so some state guidance requires higher-speed values for signs placed just after a speed change. Check approach speeds on site rather than assuming the signed limit is being followed.
What should I do if a crest hides the traffic controller?
Do not open the site as drawn. Report it to the supervisor and the TGS designer. Common responses include moving the control point back before the crest, extending advance warning, or adding a second controller or warning device, but any change must be assessed and authorised under the project's process.