Intersection Geometry

ISD from the gap-acceptance time

Choosing the time gap

The minor-road leg and the decision point

Approach triangles and the other cases

Clearing the triangle and comparing to SSD

Exam strategy

Key equations

Worked examples

Common pitfalls

• •Citing an NCEES handbook page for the ISD method — there is none. ISD, sight triangles, and time gaps come from the AASHTO Green Book; the handbook's §5.2 covers only generic curve geometry.
• •Doubling the unit conversion: if you write the speed in ft/s and then multiply by $1.47$, you convert twice. The $1.47$ goes with speed in mph only.
• •Using the left-turn gap ($7.5$ s) when the maneuver is a right turn or a crossing ($6.5$ s), or vice versa — read the maneuver before picking $t_g$.
• •Forgetting the multilane adjustment $0.5\,(n_L-1)$ s, which can add hundreds of feet to ISD on a wide arterial.
• •Confusing the approach triangle (moving vehicle, travel time) with the departure triangle (stopped vehicle, gap time) — different cases, same arithmetic but different time values.
• •Sizing the triangle for a passenger car when the design vehicle is a truck; truck gaps are larger ($0.7$ s/lane and bigger base values), so a car-based ISD can strand a combination vehicle.
• •Reporting ISD without checking it against the major road's stopping sight distance — ISD must never be less than SSD for the major-road design speed.

References

• AASHTO, A Policy on Geometric Design of Highways and Streets (Green Book, GDHS-7) — Intersection Sight Distance (Cases A–F)
• NCEES PE Civil Reference Handbook — §5.2 Horizontal Design — Provides only generic curve geometry; contains no ISD or sight-triangle method.
• AASHTO Green Book — Stopping Sight Distance criteria — Lower-bound check that available ISD must satisfy.