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Sight Distance

Draws the AASHTO intersection sight triangles for a minor approach onto a major road from one pick: departure sight distances from design speed × time gap, legs running up- and down-station along the alignment, then flags anything on the obstruction layers standing inside a triangle.

Command VDCSIGHTDIST · VDC QC tab › Site QC panel
Draws the AASHTO intersection sight triangles for a minor approach onto a major road from one pick: departure sight distances from design speed × time gap, legs running up- and down-station along the alignment, then flags anything on the obstruction layers standing inside a triangle.
One pick: two departure triangles up-/down-station, apex at the driver's-eye offset. Major-road alignment (picked) Minor approach (STOP) Driver's eye @ 20 ft offset ISD 280 FT @ 25 MPH left turn t = 7.5 s ISD 240 FT @ 25 MPH right / crossing t = 6.5 s ERROR: *WALL* in triangle clear (outside) ISD = 1.47 × V × t g, rounded up to 5 ft, add 0.5 s per extra crossed lane; legs bend with a curved alignment.
Road edges / approach   Major-road alignment + driver's eye   Sight triangle C-ANNO-SGHT / obstruction ERROR   Clear (outside)

What it does

  • Pick topology: select the major-road alignment + the intersection point; the pick's side of centerline places the driver's eye.
  • Departure ISD: ISD = 1.47 × V × t_g, rounded up to 5 ft (25 mph → 280 ft left / 240 ft right).
  • Legs along station: run up- and down-station along the alignment, so a curved major road bends the sight line with it.
  • Apex: sits at the driver's-eye offset from centerline (default 20 ft).
  • Obstruction check: entities on the obstruction layers whose extents fall inside a triangle flag as ERRORs (extents-based, conservative).
  • Triangles + ISD 280 FT labels are XData-stamped; Clear removes exactly ours.

Settings & options

  • Design speed: 20–60 combo, any value typable.
  • Time gaps: left-turn 7.5 s, right/crossing 6.5 s; add 0.5 s per extra crossed lane.
  • Driver's-eye offset: apex distance from centerline (default 20 ft).
  • Obstruction layers: wildcards *BLDG* *TREE* *LSCP* *SIGN* *WALL*, editable in settings.json.
  • Triangle layer default C-ANNO-SGHT (red); check toggle in the dialog.
  • Persisted at %APPDATA%\VDC\SightDist\settings.json (env VDC_SIGHTDIST_SETTINGS). Vertical/crest sight distance is VDCROADQC's K-value territory.

Dialog

Sight Distance dialog
Sight Distance, VDCSIGHTDIST

When to use it

  • Every driveway / street connection exhibit - one pick instead of construction lines.
  • Checking a proposed monument sign, landscaping berm, or wall against the sight triangle before it's built.

How to run it

  1. Run VDCSIGHTDIST - pick Run in the mode dialog; set the design speed, time gaps, and eye offset; click OK.
  2. Select the MAJOR road alignment, then pick the intersection point (your pick's side of centerline places the driver's eye).
  3. Both departure triangles draw with ISD labels; anything on the obstruction layers inside one flags to the palette.
  4. Repeat per approach; Clear mode removes all triangles.

Options

  • Design speed / time gaps: left turn from stop 7.5 s, right/crossing 6.5 s (AASHTO passenger car); add 0.5 s per extra crossed lane.
  • Driver's eye offset: apex distance from the major-road centerline (default 20 ft).
  • Obstruction layers: wildcards (default *BLDG*, *TREE*, *LSCP*, *SIGN*, *WALL*); edit in settings.json.

Watch out for

  • This is the HORIZONTAL check - vertical sight distance over crest curves is Road QC's K-value territory.
  • The obstruction test is extents-based and conservative: a hedge reads as its bounding box.
  • An ISD leg past the alignment's end draws what fits and flags a WARNING - carry the check onto the adjoining alignment by hand.

Related: Road QC · ADA Slope Audit · Fire Coverage