Transportation Learning HubTransportation Learning Hub · PE Civil Transportation · Area 6 of 10
Intersection Geometry — PE Civil Transportation
Intersections are where paths cross, so their geometry is mostly about seeing and sorting: sight triangles that let drivers judge gaps, layouts that separate conflicting movements in space or time, and interchanges that remove the crossing altogether.
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Intersection sight distance and sight triangles
A driver waiting on the minor road needs to see far enough along the major road to judge that a gap is big enough. That requirement draws a sight triangle: one leg runs along the minor road from the driver’s eye position back from the edge of the major road, and the other runs along the major road for the intersection sight distance. Anything inside the triangle — vegetation, fences, parked vehicles, embankments — is an obstruction to be removed or set back.
\[\text{ISD} = 1.47 \, V \, t_{g}\]
| V | major-road design speed, mph |
|---|---|
| tg | time gap for the manoeuvre, s — given in exam problems; longer for left turns and for larger design vehicles than for right turns by a passenger car |
| Approach triangles | used where drivers approach without stopping (yield or no control): both drivers need to see each other in time to react |
| Departure triangles | used at stop-controlled approaches: the stopped driver needs the ISD along the major road in each direction the manoeuvre crosses or enters |
ISD is not SSD. Stopping sight distance lets a moving driver stop for an object; intersection sight distance gives a stationary driver a usable gap — different question, different formula, and the stems are written to test whether you can tell them apart.
Worked example Intersection sight distance and the departure triangle
Given: a stop-controlled minor road meets a major road with design speed \(V = 45\) mph. The time gap for the manoeuvre is given as \(t_{g} = 6.5\) s. Find the required intersection sight distance and describe the departure sight triangle.
Solution:
- \(\text{ISD} = 1.47 \times 45 \times 6.5 = 429.98 \approx 430\) ft.
- Major-road leg: 430 ft measured along the major road from the conflict point, in the direction the manoeuvre must judge.
- Minor-road leg: from that same conflict point back to the driver’s eye position on the minor road, set back from the edge of the major-road travelled way by the setback given in the problem.
- The triangle between those legs must stay clear of obstructions.
Answer: ISD \(\approx 430\) ft; the departure triangle has a 430-ft major-road leg and a minor-road leg ending at the driver’s eye position.
Interchanges
An interchange removes crossing conflicts by grade separation and replaces them with merge and diverge manoeuvres on ramps. At an entrance, the design problem is acceleration: the ramp must give the entering vehicle room to reach near mainline speed and a taper or parallel lane in which to merge. At an exit, it is deceleration: the diverging vehicle should leave the through lanes at speed and slow on the ramp, not in the through lane. Where an entrance is closely followed by an exit, their merge and diverge areas overlap into a weaving section whose length and lane balance control its operation. Ramp horizontal and vertical design reuses the earlier areas — curve elements and superelevation from Horizontal Design, profile and sight distance from Vertical Design — applied at ramp design speeds. Common forms (diamond, cloverleaf, directional) differ mainly in how left-turning ramp movements are handled and how much land and structure they consume.
At-grade layout and roundabouts
At-grade design sorts conflicts in space: channelising islands that fix turning paths, adequate turning radii for the design vehicle without inviting car speeds, median refuges that let a crossing be made in two stages, and corner sight triangles kept clear. A roundabout sorts them by geometry instead of signals: entering traffic yields to circulating traffic, and entry deflection — the entering path is deliberately bent around the central island — forces low entry speeds, which is what converts potential high-energy crossing conflicts into low-speed merging ones. The splitter island on each approach provides that deflection and a pedestrian refuge; the circulatory roadway carries the one-way flow around the island; and truck aprons let long vehicles track over the island edge without weakening the deflection for cars.