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Transportation & Surveying

Stopping Sight Distance (AASHTO)

AASHTO stopping sight distance: brake-reaction distance plus braking distance, with grade adjustment, reporting the exact computed SSD and the design value rounded up per the Green Book tables (next 5 ft US / next 5 m metric). Defaults: 2.5 s reaction time, 11.2 ft/s² (US) / 3.4 m/s² (metric) deceleration. Use for: 'find the SSD for a design speed of 60 mph', grade-adjusted stopping distance, checking Green Book Table 3-1 values. Not for: passing sight distance (passing-sight-distance) or decision sight distance.

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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

  1. 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

  2. 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

  3. 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

  4. 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

  5. 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)

570ft

Where this answer was checked
source
Kinematics + 2011 AASHTO Table 3-1 via MDOT Sight Distance Guidelines (fetched): https://www.gcrc.org/forms-documents/forms/driveway-policy/168-mdot-sight-distance-guidelines/file — SSD = v·t + v²/(2a); published row: 60 mph → 220.5 / 345.5 / 566.0 calc / 570 design
verified by
both
derivation
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

  1. 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

  2. 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

  3. 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

  4. 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

  5. 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)

185m

Where this answer was checked
source
Kinematics + metric AASHTO Table 3-1 via INDOT Design Manual Ch. 42 Fig. 42-1A (fetched): https://www.in.gov/dot/div/contracts/standards/dm-Archived/09Metric/Part5/Ch42/DCh42.pdf — published row: 100 km/h → 69.5 / 114.7 / 184.2 calc / 185 design
verified by
both
derivation
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

  1. 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

  2. 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

  3. 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

  4. 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

  5. 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)

305ft

Where this answer was checked
source
Kinematics + 2011 AASHTO Table 3-1 via MDOT Sight Distance Guidelines (fetched): https://www.gcrc.org/forms-documents/forms/driveway-policy/168-mdot-sight-distance-guidelines/file — published row: 40 mph → 147.0 / 153.6 / 300.6 calc / 305 design
verified by
both
derivation
v = 40 mph = 58.667 ft/s; default a = 11.2 ft/s². dPR = 58.667·2.5 = 146.67 ft. db = 58.667²/(2·11.2) = 3441.8/22.4 = 153.65 ft. SSD = 300.32 ft. Table basis 1.47·40·2.5 + 1.075·1600/11.2 = 147.0 + 153.57 = 300.57 (published calc 300.6) → design 305 ft.