Transportation Learning HubFree calculator
Stopping Sight Distance Calculator — PE Civil Transportation Worked Example
Stopping sight distance is the sum of the distance travelled while the driver perceives and reacts, and the distance needed to brake to a stop. Enter the design speed, perception–reaction time, friction factor and grade (signed — negative for a downgrade) and check your hand calculation.
For study and checking only. This calculator runs entirely in your browser — nothing is sent anywhere. It is meant to verify your hand calculations while you study; on the exam you will work by hand with the NCEES reference handbook.
Quick answer: the two-part SSD formula
\[d_{pr} = 1.47\,V\,t \qquad d_{b} = \frac{V^{2}}{30\,(f \pm G)}\]
The perception–reaction distance uses the 1.47 conversion from mph to ft/s. In the braking term, the grade enters as a decimal: add it for an upgrade, subtract it for a downgrade — a downgrade lengthens the braking distance, which is exactly the sign error the exam plants.
How to use it — and how to sanity-check the answer
Set the four inputs from the problem statement — on the exam, the reaction time and friction factor (or deceleration) are given in the stem. Sanity-check the output in two parts: the reaction distance should be roughly 1.5 × speed × time, and the braking distance should grow with the square of speed. If a downgrade gives you a shorter total than the level-grade case, the grade sign is wrong.
Worked example (reproduces in the tool above)
Worked example SSD on a downgrade
Given: A highway has a design speed of 45 mph on a 4% downgrade, with \(t = 2.5\) s and \(f = 0.35\).
Solution:
- Perception–reaction: \(d_{pr} = 1.47 \times 45 \times 2.5 = 165.4\) ft.
- Braking on the downgrade: \(d_{b} = 45^{2} / [30 \times (0.35 - 0.04)] = 2{,}025 / 9.3 = 217.7\) ft.
- Total: \(\text{SSD} = 165.4 + 217.7 = 383.1 \approx 383\) ft.
Answer: SSD \(\approx 383\) ft.