Reference
Sources
Engineering values on this site are derived from primary documents — federal publications and official agency design manuals — not from other calculators. This page lists every document relied on and what each one supports.
How sources are ranked
Where documents disagree, we follow this order and say so explicitly rather than silently picking one:
- Official federal transportation publications
- Standards bodies, where lawfully available
- Official state DOT design manuals
- Reputable academic and technical publications
This site is not an official AASHTO calculator and is not endorsed by any agency. Where a methodology originates with AASHTO, we describe it as AASHTO-based methodologyas adopted and published by the agency whose manual we worked from, because that is what the documents we hold actually establish.
Documents
- Federal publication
Speed Concepts: Informational Guide
Federal explanation of the assumptions behind stopping sight distance criteria: the 2.5 s perception-reaction time and its relationship to the 90th percentile driver, the 11.2 ft/s² (0.35g) deceleration rate and its wet-pavement basis, driver eye height, and the mean-driver comparison model (1.1 s, 17.71 ft/s²).
- State DOT manual
Highway Design Manual, Chapter 200 — Geometric Design and Structure Standards
California adopted minimum stopping sight distance standards (Table 201.1), the 20 percent increase on sustained downgrades (Index 201.3), and the 3.5 ft driver eye height / 0.5 ft object height criteria.
- State DOT manual
Indiana Design Manual, Chapter 44 — Vertical Alignment
The four crest and sag vertical curve length equations for both S < L and S ≥ L, printed with their derivation: the crest constant 2158 as 100(√(2h₁)+√(2h₂))² for a 3.5 ft eye height and 2 ft object height, and the sag headlight criterion assuming a 2 ft headlight height and a 1 degree upward divergence of the beam.
- State DOT manual
Roadway Design Manual
Stopping sight distance equations for level and graded roadways, the 2.5 s brake reaction time and 11.2 ft/s² deceleration rate, and the calculated and adopted design SSD values in Tables 4-23 and 4-24.
- State DOT manual
Design Manual M 22-01, Chapter 1260 — Sight Distance
Independent corroboration of the AASHTO-derived stopping sight distance on grades table, used to resolve a discrepancy in the TxDOT Table 4-24 30 mph upgrade row.
Documented discrepancies
Published tables occasionally disagree with the equations printed in the same document. Where we have found this, we record it here and surface it in the calculator at the input that triggers it, rather than quietly correcting the source.
TxDOT Table 4-24 — 30 mph, 3% upgrade
Published 200 ftEquation 190 ftCorroborated 190 ft
TxDOT Table 4-24 prints 200 ft for 30 mph on a 3% upgrade, in both the PDF and the current web edition of §4.11.1. The grade equation given in the same section yields 189.7 ft, which rounds to 190 ft. The published value also breaks the monotonic pattern of its own row: the 6% and 9% upgrade values are 184 and 179 ft, so a 3% upgrade value of 200 ft would mean the first 3% of upgrade increases the required distance above the level design value of 200 ft, which is the opposite of the physical effect. AASHTO-derived tables in other state manuals, including WSDOT Design Manual Chapter 1260, publish 190 ft for this cell. This calculator reports the equation result and flags the published value.
TxDOT Table 4-24 — 15 mph, 9% downgrade
Published 86 ftEquation 84 ft
The November 2024 PDF edition prints 85 ft where the current TxDOT web edition prints 86 ft. The grade equation yields 84.2 ft. Values at 15 mph sit at the bottom of the design speed range where the published table departs from the equation by 1 to 2 ft throughout.