Calculator inputs
This page keeps sections stacked vertically, while the input controls flow in three columns on large screens, two on medium screens, and one on mobile.
Example data table
| Scenario | Passing Speed (km/h) | Passed Speed (km/h) | t1 (s) | t2 (s) | Acceleration (km/h/s) | d3 (m) | Estimated PSD (m) |
|---|---|---|---|---|---|---|---|
| Lower-speed rural pass | 70 | 55 | 4.0 | 9.3 | 2.25 | 30 | 397.79 |
| Moderate-speed design case | 85 | 65 | 4.3 | 10.7 | 2.37 | 75 | 580.19 |
| Higher-speed overtaking case | 100 | 75 | 4.5 | 11.3 | 2.41 | 90 | 714.18 |
Formula used
d1 = 0.278 × t1 × [Vb + (a × t1 / 2)]
d2 = 0.278 × V × t2
d4 = (2/3) × d2 or d4 = 0.278 × Vo × f × t2
Base PSD = d1 + d2 + d3 + d4
Final PSD = Base PSD × (1 + Safety Factor / 100)
Where:
V = average passing vehicle speed in km/h
Vb = passed vehicle speed in km/h
Vo = opposing vehicle speed in km/h
t1 = initial maneuver time in seconds
t2 = time spent in the opposing lane in seconds
a = average acceleration in km/h/s
d3 = end clearance distance in meters
f = exposure factor for the opposing vehicle component
How to use this calculator
Enter the passing speed, the slower vehicle speed, and the time assumptions for the maneuver. Use measured or design values that match the project context.
Set the clearance distance and choose how you want to estimate the opposing-vehicle portion. Use the simplified method for quick checks, or manual speed with exposure for more control.
Select the roadway mode. For two-way roads, the calculator includes the opposing vehicle distance. For divided or one-way conditions, it removes that component automatically.
Press the calculate button. Review the total passing sight distance, study the component chart, and export the results as CSV or PDF for documentation.
Frequently asked questions
1. What does passing sight distance mean?
It is the visible roadway length needed for a driver to pass a slower vehicle and return safely before meeting opposing traffic or closing an unsafe gap.
2. Why are t1 and t2 separated?
They describe different phases. t1 covers reaction and initial maneuvering. t2 covers the time the passing vehicle actually occupies the opposing lane.
3. When should I use the simplified d4 method?
Use it for fast design checks when you want a conventional estimate for opposing traffic distance without entering a separate opposing speed.
4. When is manual d4 better?
Manual d4 is useful when field speeds differ from the passing speed or when you want to test different exposure assumptions for opposing traffic.
5. Does this calculator work for divided highways?
Yes, as a planning aid. Choose divided or one-way mode and the tool removes the opposing vehicle component from the total requirement.
6. Why add a safety factor?
A safety factor creates extra margin above the base computation. It can help during concept studies, sensitivity checks, or conservative preliminary design reviews.
7. Should grade or driver behavior change my inputs?
Yes. Uphill passing, vehicle performance limits, heavy vehicles, and local driving behavior can all change acceleration, maneuver time, and clearance assumptions.
8. Is this enough for final roadway design approval?
No. It is a strong engineering calculator, but final design should still follow the governing agency manual, project standards, and professional review requirements.