Stopping Sight Distance Calculator: Reaction Distance, Braking Distance and Braking Time
How far a vehicle travels before it stops.
Engineering student calculators
Free textbook calculators with every step shown, so you can check your own working. Pick one, type in your numbers and see the formulas worked out. Or switch to Ask a question for an explained answer. No account needed.
Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.
How this calculator works
Enter the speed and the perception-reaction time.
Enter the coefficient of friction (or the deceleration) and the grade of the road, with uphill as a positive number.
The calculator gives the reaction distance, the braking distance, the stopping sight distance and the braking time.
Formulas used
- SSD = vt + \frac{v^2}{2g(f \pm G)}
- BD = \frac{v^2}{2g(f \pm G)}
- a = g(f \pm G)
- t_b = \frac{v}{g(f \pm G)}
- B_e = \frac{f}{f_{ave}} \times 100\%
Worked example
With the default numbers in the calculator above:
- Speed in metres per second: 27.78 m/s
- Deceleration: 3.43 m/s2
- Perception-reaction distance: 69.44 m
- Braking distance: 112.36 m
- Stopping sight distance: 181.81 m
- Braking time: 8.09 s
- Total time to stop: 10.59 s
Change the numbers in the calculator to see the full working, every step, for your own case.
A second worked example
A different case, with these numbers: Speed of the vehicle (in the unit chosen above) 100; Perception-reaction time 0 s; Coefficient of friction between the tyres and the road 0.6; Grade of the road 2 %.
- Speed in metres per second: 27.78 m/s
- Deceleration: 6.08 m/s2
- Perception-reaction distance: 0.00 m
- Braking distance: 63.43 m
- Stopping sight distance: 63.43 m
- Braking time: 4.57 s
- Total time to stop: 4.57 s
Common questions
What is stopping sight distance?
The distance a vehicle travels from the moment the driver sees a hazard to the moment it stops. It is the distance travelled during the reaction time plus the braking distance.
Is the grade added or subtracted?
Uphill grades help the braking, so the grade is added to the coefficient of friction. Downhill grades work against it, so the grade is taken as negative.
What reaction time should I use?
Road design standards set the value to use. 2.5 seconds is a commonly used design value. Use 0 to find the braking distance alone.
Related calculators
Got a question about this? Ask an engineering question and get a plain-language answer with references.
Further reading: Any highway engineering text (stopping sight distance). Use the road design standard that applies (for example Austroads) for the design values.
Need a calculator we do not have?
Embed this calculator on your own page
Copy this code into your page. The calculator runs inside the frame and links back to Drafting Central.
Need this drawn, designed or checked?
Drafting Central is a mechanical engineering and drafting business working with customers across Australia and around the world. A calculator gives you the numbers; we can turn them into drawings and designs.
Need help with a real project?
Talk to Drafting Central →