Calculate reaction and braking distance with road grade adjustment. See computed stopping sight distance and the rounded AASHTO-based value.
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Worked examples
These are the solver's own reference problems — the answers come from published sources or independent hand computation, never from the solver itself. The solution below is the live solver output for each.
Example 1
US 60 mph, t = 2.5 s, a = 11.2 ft/s², level → 565.7 ft exact, 570 ft design
Given
design speed
60 mi/h
reaction time
2.5 s
deceleration
11.2 ft/s^2
Assumptions
AASHTO stopping-sight-distance model: constant deceleration from the design speed after a fixed brake-reaction time (design defaults 2.5 s and 11.2 ft/s² US / 3.4 m/s² metric).
US design convention: the design SSD is the Green Book table basis (rounded-constant form) rounded UP to the next 5-ft increment, reproducing AASHTO Table 3-1.
Solution steps
Brake-reaction distance
Distance covered at full speed while the driver perceives the hazard and applies the brakes.
dPR = v·t
dPR = 60 mi/h = 88 ft/s; dPR = 88 ft/s × 2.5 s = 220 ft
= 220 ft
Braking distance
Kinematics of a constant-deceleration stop from the design speed on a level road.
db = v²/(2·(a + g·G/100)), G in percent (0 on level ground)
db = 88 ft/s² / (2 × 11.2 ft/s^2) = 345.7 ft
= 345.7 ft
Stopping sight distance (exact)
The sum of the brake-reaction and braking distances, from the exact kinematics.
SSD = dPR + db
SSD = 220 ft + 345.7 ft = 565.7 ft
= 565.7 ft
AASHTO table basis (handbook form)
The Green Book's US form uses rounded constants (1.47 ≈ 5280/3600 mph→ft/s, and 1.075; on grades V²/(30((a/32.2) ± G/100))) — which is why table values differ from the exact physics by a fraction of a percent.
SSD = 1.47·V·t + 1.075·V²/a (level) or V²/(30((a/32.2) ± G/100)) on grades; V in mph, a in ft/s²
Table basis ≈ 566 ft vs exact 565.7 ft
Design value
AASHTO rounds the table basis UP to the next 5-ft (US) or 5-m (metric) increment for design use.
SSD_design = table basis rounded up to the next 5-unit increment
SSD_design = 570 ft
= 570 ft
Results
Brake-reaction distance
220ft
Braking distance
345.7ft
Stopping sight distance (computed, exact)
565.7ft
Design stopping sight distance (rounded up to 5 ft)
v = 60 mph = 88.00 ft/s exactly. dPR = 88·2.5 = 220.0 ft. db = 88²/(2·11.2) = 7744/22.4 = 345.71 ft. SSD = 565.71 ft → design (next 5 ft) = 570 ft. The published 'calculated' 566.0 uses the rounded handbook constants (1.47, 1.075); both round up to the same 570 ft design value.
Example 2
SI 100 km/h, AASHTO defaults → 182.9 m exact, 185 m design
Given
design speed
100 km/h
Assumptions
AASHTO stopping-sight-distance model: constant deceleration from the design speed after a fixed brake-reaction time (design defaults 2.5 s and 11.2 ft/s² US / 3.4 m/s² metric).
Metric design convention: the design SSD is the Green Book table basis (rounded-constant form) rounded UP to the next 5-m increment, reproducing the metric AASHTO Table 3-1.
Solution steps
Brake-reaction distance
Distance covered at full speed while the driver perceives the hazard and applies the brakes.
dPR = v·t
dPR = 100 km/h = 27.78 m/s; dPR = 27.78 m/s × 2.5 s = 69.44 m
= 69.44 m
Braking distance
Kinematics of a constant-deceleration stop from the design speed on a level road.
db = v²/(2·(a + g·G/100)), G in percent (0 on level ground)
db = 27.78 m/s² / (2 × 3.4 m/s^2) = 113.5 m
= 113.5 m
Stopping sight distance (exact)
The sum of the brake-reaction and braking distances, from the exact kinematics.
SSD = dPR + db
SSD = 69.44 m + 113.5 m = 182.9 m
= 182.9 m
AASHTO table basis (handbook form)
The Green Book's metric form uses rounded constants (0.278 ≈ 1/3.6 km/h→m/s, and 0.039; on grades V²/(254((a/9.81) ± G/100))) — which is why table values differ from the exact physics by a fraction of a percent.
SSD = 0.278·V·t + 0.039·V²/a (level) or V²/(254((a/9.81) ± G/100)) on grades; V in km/h, a in m/s²
Table basis ≈ 184.2 m vs exact 182.9 m
Design value
AASHTO rounds the table basis UP to the next 5-ft (US) or 5-m (metric) increment for design use.
SSD_design = table basis rounded up to the next 5-unit increment
SSD_design = 185 m
= 185 m
Results
Brake-reaction distance
69.44m
Braking distance
113.5m
Stopping sight distance (computed, exact)
182.9m
Design stopping sight distance (rounded up to 5 m)
v = 100/3.6 = 27.778 m/s. dPR = 27.778·2.5 = 69.44 m. db = 27.778²/(2·3.4) = 771.60/6.8 = 113.47 m. SSD = 182.92 m → design (next 5 m) = 185 m, matching the published design value. The published calculated 184.2 m uses the rounded 0.039 metric constant (exact 1/(2·3.6²·3.4) route gives 113.47 not 114.7); the design value is identical.
Example 3
US 40 mph, all defaults → 300.3 ft exact, 305 ft design
Given
design speed
40 mi/h
Assumptions
AASHTO stopping-sight-distance model: constant deceleration from the design speed after a fixed brake-reaction time (design defaults 2.5 s and 11.2 ft/s² US / 3.4 m/s² metric).
US design convention: the design SSD is the Green Book table basis (rounded-constant form) rounded UP to the next 5-ft increment, reproducing AASHTO Table 3-1.
Solution steps
Brake-reaction distance
Distance covered at full speed while the driver perceives the hazard and applies the brakes.
dPR = v·t
dPR = 40 mi/h = 58.67 ft/s; dPR = 58.67 ft/s × 2.5 s = 146.7 ft
= 146.7 ft
Braking distance
Kinematics of a constant-deceleration stop from the design speed on a level road.
db = v²/(2·(a + g·G/100)), G in percent (0 on level ground)
db = 58.67 ft/s² / (2 × 11.2 ft/s^2) = 153.7 ft
= 153.7 ft
Stopping sight distance (exact)
The sum of the brake-reaction and braking distances, from the exact kinematics.
SSD = dPR + db
SSD = 146.7 ft + 153.7 ft = 300.3 ft
= 300.3 ft
AASHTO table basis (handbook form)
The Green Book's US form uses rounded constants (1.47 ≈ 5280/3600 mph→ft/s, and 1.075; on grades V²/(30((a/32.2) ± G/100))) — which is why table values differ from the exact physics by a fraction of a percent.
SSD = 1.47·V·t + 1.075·V²/a (level) or V²/(30((a/32.2) ± G/100)) on grades; V in mph, a in ft/s²
Table basis ≈ 300.6 ft vs exact 300.3 ft
Design value
AASHTO rounds the table basis UP to the next 5-ft (US) or 5-m (metric) increment for design use.
SSD_design = table basis rounded up to the next 5-unit increment
SSD_design = 305 ft
= 305 ft
Results
Brake-reaction distance
146.7ft
Braking distance
153.7ft
Stopping sight distance (computed, exact)
300.3ft
Design stopping sight distance (rounded up to 5 ft)