Stopping Sight Distance Calculator
Enter speed, perception-reaction time, braking deceleration, and signed grade. Positive grade is uphill; negative grade is downhill.
A QUICK WALKTHROUGH
How to use this tool
- Choose speed units and enter a finite speed greater than zero.
- Enter nonnegative reaction time, positive braking deceleration, and signed grade percentage (uphill positive, downhill negative).
- Calculate and review reaction distance, braking distance, and total; copy or download the report.
Formula and units
v = speed converted to m/s. Reaction distance = v × t. Effective braking deceleration = a + g × grade/100, where g = 9.80665 m/s² and grade is positive uphill, negative downhill. Braking distance = v² ÷ (2 × effective deceleration). Total stopping distance = reaction distance + braking distance.
Scope and assumptions
A simplified physics estimate using your inputs, not a regulatory default, roadway design value, or safety determination. It omits perception variability, brake response, tire-road friction variation, vehicle condition, visibility, and site geometry. Do not substitute it for applicable standards or qualified road design.
Reference
FHWA, stopping distance includes perception-reaction distance and braking distance; grade modifies braking distance: https://www.fhwa.dot.gov/publications/research/safety/ihsdm/02045/3.cfm . This tool uses user inputs and does not adopt regulatory assumptions.
GOOD TO KNOW
Common questions
Does this use regulatory default values?
No. Speed, reaction time, deceleration, and grade are user inputs. The result is not a roadway design or compliance value.