ISD ≠ SSD. Stopping sight distance is the along-the-road minimum every road must give a driver to stop; intersection sight distance is what a driver at a minor approach needs to SEE along the major road to depart safely. ISD is almost always longer. Meeting SSD but failing ISD is a real (and common) condition — flag both separately.
The departure sight triangle (stop control): vertex at the minor-road driver’s eye, legs along the minor approach setback and along the major road. The Green Book builds ISD from a time gap (seconds a departing driver needs) converted to distance at the major-road design speed — gap values differ by maneuver and vehicle, with adjustments for grades and added lanes crossed #status/verify gap values & adjustments in current Green Book edition; FDM section for SHS criteria. Three cases you’ll actually run:
- Left turn from stop — longest gap (cross near lanes + accelerate into far side); governs most driveways onto divided roads (check each roadway of a divided highway separately; the median may store the design vehicle or not).
- Right turn from stop — shorter gap; look-left leg only, but don’t forget bikes/peds approaching from the right.
- Crossing maneuver — full crossing of all lanes; governs at full-median openings and 4-leg minor crossings.
Eye/object conventions: driver eye height and the opposing vehicle object height are set by the Green Book (passenger-car values; higher for trucks as the design vehicle), and the driver’s decision point sits a set distance back from the edge of the traveled way — use the book’s values, not memory #status/verify heights & setback distance. This is why a 2-ft wall can pass and a 4-ft hedge can’t.
Field measurement (two-person): mark the decision point on the minor approach at the specified setback; observer holds eye-height at that point, rodman walks the major road each direction with a target at object height; record the distance where the target stays continuously visible; repeat both directions, photograph the limiting obstruction. Compare measured vs required at the appropriate speed — posted, design, or measured operating speed per the reviewing agency #status/verify which speed basis your agency requires.
Warning
The classic review comment: landscaping plans and monument signs are drawn by people who never ran ISD. Plot the sight triangles ON the landscape/signage plan and call out max mature height inside them. “Clear sight triangle to be maintained” as a note without a drawn triangle is an unenforceable comment.
Tip
Speed input shaky (no posted limit, rural transition, “everyone does 50”)? A spot-speed study — or FHWA’s free USLIMITS2 (highways.dot.gov/safety/speed-management/uslimits2) for the speed-zoning question behind it — beats arguing about the right V. See Speed Limits in Florida.
Example
Driveway (left turn out) onto a 45 mph 4-lane divided road: required ISD = gap time for the left-turn case (plus added-lane adjustment) × 1.47 × 45
#status/verify— measure available along the near roadway from the decision point; if a proposed monument sign at the corner clips the triangle, the fix is relocate/lower the sign or restrict the driveway to right-in/right-out. Write all three options in the comment.
Practice question: stop-controlled minor street, crest curve 400 ft left of the intersection limits sight to 380 ft, required ISD ≈ 500 ft — list three fixes in order of cost and which one changes the control type. QA/QC reminder: every ISD table states its speed basis, case, edition, and eye/object heights — an unlabeled “sight distance OK” is not a finding.
See also: AASHTO Green Book · Turn Lane and Queue Checks · Speed Limits in Florida