Superelevation & widening

Banking the road into curves — building up gradually, the way it does on a real road — and widening the pavement on tight curves.

What superelevation is

On a curve the pavement is tilted (banked) toward the inside so gravity carries part of the centripetal force instead of tyre friction. The tilt is the superelevation rate e (a cross-slope, e.g. 6%). The maximum allowed e_max comes from the active standard (AASHTO 8%, RAL 6%, VSS 7%, KGM 8%).

It is gradual — the runoff

A road cannot jump from a flat crown to full bank at the start of a curve. The change happens over a transition: tangent runout (outer lane rotates from normal crown to level) then superelevation runoff L_r (level to full e). yucad3d builds the proper diagram (AASHTO “Method 2”): zero on the tangents, a gradual ramp through the transitions, full cant through the circular curve. Along a clothoid spiral the cant ramps with the spiral; at a direct tangent↔curve boundary the runoff is split ~2/3 on the tangent, 1/3 into the curve.

Turning it on

Superelevation is opt-in — curves keep their design crown until you enable it. In the Superelevation tab (Assembly Designer / Composer):

• Auto superelevation from curvature — turns on the standard-aware runoff diagram.
• Standard + Design speed — AASHTO / RAL / VSS / KGM and a speed (20–140 km/h). They drive e_max, the design rate per radius, and the runoff length — so the banking is realistic for the chosen rules.
• The cross-slope table lists the per-lane / per-shoulder design slopes (the crown), each with an “enabled in preview” tick.

Seeing it react

With auto-super on, drag the station slider in the 2D preview: flat on the tangent, tilting through the runoff, full bank in the curve. The preview writes the live grade % on each link. The 3D preview shows the whole corridor banking continuously along the curve.

The Superelevation Editor (per corridor)

Tools › Superelevation Editor sets a manual override diagram on a real corridor, or computes one:

• A (Station, Rate %) table — Add Row / Remove Row. The values are linearly interpolated and clamped at the ends; a non-empty table replaces the automatic banking. Reset to Auto empties it.
• Compute Runoff — pick Standard, Speed, e_max, lane width and number of lanes, and it fills the table with the Method-2 runoff control points (and reports R_min).

An applied override shows up on the corridor itself: the Properties panel's corridor summary reports the control-point count and the peak rate (e.g. Superelevation: 8 control point(s), peak e 5.8%), so you can confirm the design took without reopening the editor.

Curve widening

Tight curves need extra width because long vehicles track inside their lane. Tools › Curve Widening Editor works like the Superelevation Editor: a (Station, Widening m) table you can edit by hand, plus Compute Widening (Standard, Speed, number of lanes) that fills it from the widening transition. The widening is split each side; the shoulders translate out, and it tapers over a transition length the same way superelevation runs off.