Stopping sight distance (brake reaction + braking distance, with grade correction) and decision sight distance for avoidance maneuvers A to E, per the AASHTO Green Book (Tables 3-1, 3-2 and 3-3).
SSD = 0.278·V·t + 0.039·V²/a (AASHTO Eq. 3-2), V in kph, t in s, a in m/s².
On a grade: SSD = 0.278·V·t + V² / [254·((a/9.81) ± G)] (Eq. 3-3), G as a decimal
(+ ascending / upgrade, − descending / downgrade). Design values are the calculated distance rounded up to the next 5 m.
The grade changes only the braking part; the brake reaction distance is the same in both directions.
Verified: V = 100 kph, t = 2.5 s, a = 3.4 m/s² → 69.5 + 114.7 = 184.2 → SSD = 185 m ✓ (Table 3-1)
Calculated with the t and a entered on the left (AASHTO defaults reproduce Table 3-1). highlighted = current selection.
Stopping sight distance is the minimum that must be available at every point on every road, measured from a
driver's eye height of 1.08 m to an object 0.60 m high. It governs crest and sag vertical curves and horizontal sight lines.
Decision sight distance should be provided where drivers must make complex or unexpected decisions: interchange and
intersection approaches, lane drops, changes in cross-section, toll plazas, and areas with visual clutter or
concentrated demand for information. Where DSD cannot be provided, advance warning signs and markings should be considered.
Stopping sight distance is the sum of two parts: the brake reaction distance travelled while the driver perceives the hazard and applies the brakes (0.278·V·t), and the braking distance while the vehicle slows to a stop (0.039·V²/a). AASHTO uses t = 2.5 s and a = 3.4 m/s², and rounds the result up to the design values of Table 3-1.
Gravity helps braking on an upgrade and hinders it on a downgrade. AASHTO's grade formula replaces the braking term with V² / [254·((a/9.81) ± G)], where G is the grade as a decimal. On two-way roads without a median, the longer (downgrade) distance normally governs, because the same curve is used by traffic in both directions.
Stopping sight distance lets a driver stop for an object in the road. Decision sight distance gives the extra time needed to detect an unexpected or hard-to-perceive situation, recognise the hazard, choose a speed and path, and complete the manoeuvre. It is typically 1.5 to 2.5 times the stopping sight distance.
A: stop on a rural road (t = 3.0 s). B: stop on an urban road (t = 9.1 s). C: speed/path/direction change on a rural road (t = 10.2–11.2 s). D: speed/path/direction change on a suburban road (t = 12.1–12.9 s). E: speed/path/direction change on an urban road (t = 14.0–14.5 s). Urban conditions need longer times because drivers face more information and traffic.
The calculator follows A Policy on Geometric Design of Highways and Streets, 7th Edition (2018), Section 3.2. The stopping and decision sight distance criteria (t = 2.5 s, a = 3.4 m/s², and the Table 3-3 avoidance maneuvers) have been unchanged since the 2001 edition. Always confirm against the edition adopted by your road authority.