Safe Approach Speed Calculator

Estimate the maximum safe speed a vehicle can use to approach a yield-controlled or uncontrolled intersection, based on AASHTO sight distance criteria.

Site Information

Date

Analyst

Subdivision (optional)

Township

Major road name

Minor road name

Posted speed limit of major road (mph)

Measured Values

Width of roads (edge of metal / edge of pavement)

Major road width (ft)

Minor road width (ft)

Distance to obstruction (ft) from E/M or E/P

a and d measure from the major road centerline; b and c measure from the minor road.

a (major road)

b (minor road)

c (minor road)

d (major road)

Intersection geometry

Delta (degrees, counter-clockwise)

Assumed Values

Defaults mirror the worksheet screenshot (perception/reaction time from AASHTO). Adjust if your analysis uses different values.

Factor of safety for speed (mph)

Offset from centerline to driver eye, O (ft)

Perception / reaction time, t (sec)

Deceleration rate, A (ft/s²)

Clearance distance beyond SSD (AAA EC) (ft)

Safe approach speed (mph)

*Clearance distance currently not used in the legacy sheet; keep blank unless your analysis calls for it.

**10 mph has been the generally accepted safe approach speed; adjust as needed.

Stop or Yield field summary

Use this section for the PDF/printout. All fields mirror the form inputs plus the calculated safe approach speeds.

Site information

Date	—
Analyst	—
Subdivision	—
Township	—
Major road	—
Minor road	—
Posted speed (mph)	—

Measured values

Major road width (ft)	—
Minor road width (ft)	—
a (ft)	—
b (ft)	—
c (ft)	—
d (ft)	—
Delta angle (°)	—

Assumed values

Safety factor (mph)	—
Driver eye offset O (ft)	—
Perception / reaction t (sec)	—
Deceleration A (ft/s²)	—
Safe approach speed (mph)	—

Calculated values

V₁ (mph)	—
D₁ (SSD, ft)	—
D₂a (ft)	—
D₂c (ft)	—

Sight triangle diagram with major/minor roads and obstruction offsets

Reference layout

Clear View Triangle Diagram

Sight triangle diagram with major/minor roads, obstruction offsets, and labeled distances a, b, c, d

The provided CAD-style diagram mirrors the worksheet markup so crews can compare the calculator inputs against the field sketch.

Next steps

Still on deck for this tool:

Clear-view triangle check vs. posted speed (flag when sight distance is short).

The printable summary above is now live—hit “Print summary” once your data is locked in.