Sight Distance Calculator

In the braking-distance term V²/(30(f ± G)), gravity works with the brakes on an uphill grade (+G adds to the denominator), so the vehicle sheds speed faster and needs less pavement to stop. On a downhill grade, gravity fights the brakes (−G subtracts from the denominator), so the same friction coefficient has to arrest more energy over a longer distance. That's why this calculator lets grade run from -10% to +10% and folds it directly into the braking-distance denominator.

Stopping sight distance uses a physics-based formula built from reaction time and braking friction, but passing sight distance depends on harder-to-model factors like the speed of the vehicle being passed and how fast opposing traffic is closing. Rather than replicate the full AASHTO Green Book passing-sight-distance procedure, this calculator uses a simple empirical fit (8.5 × design speed) that tracks the Green Book's published values reasonably well but should be treated as a planning-level estimate rather than a final design number.

This calculator adds roughly 4.5 seconds on top of your entered perception-reaction time before applying the same braking-distance term used for SSD. That extra time represents a driver who has to recognize an unexpected or complex situation — like a poorly marked exit — and decide on a response, not just brake reflexively, so decision sight distance always comes out longer than stopping sight distance for the same speed and friction inputs.

It compares your calculated stopping sight distance against the AASHTO Green Book Table 3-1 minimum SSD for your entered design speed and flags the result as adequate if it falls within 10% of that tabulated minimum. That table comes from the AASHTO Green Book, 7th Edition (2018) — the same edition state DOT design manuals such as New York's Highway Design Manual (Chapter 2, §2.6.6) reference for stopping sight distance design. It's a quick sanity check against known standard values, not a substitute for verifying your specific alignment against the current Green Book table for your exact design speed.