Geometric design sets lane width, curves, and sight distance so drivers can see, steer, and stop safely.

Scope (geometric design)

This page covers horizontal and vertical alignment, cross-section (lanes, shoulders, verge), and sight distance so that drivers can navigate, see hazards, and stop within available space. In South Africa, national roads typically follow SANRAL TRH documents; other roads reference COLTO and municipal standards. Use the SSD calculator as a concept check—match perception–reaction time, deceleration, and grade corrections to your code tables and project criteria.

Stopping sight distance (concept)

  1. Choose design speed V (km/h) and a perception–reaction time t (s) consistent with your brief (often ~1.5–2.5 s for SSD).
  2. Select a comfortable longitudinal deceleration a (m/s²)—typical illustrative values are often a few m/s²; verify against code.
  3. Convert speed to m/s (the calculator uses V/3.6) and compute SSD ≈ vt + v²/(2a); add grade and wet-weather factors in full design.
  4. Check horizontal and vertical curves so that sight obstructions (barriers, cut slopes) do not reduce available sight below required SSD.

Transportation Engineering — Geometric Design

Plan, profile, and cross-section: align the road so vehicles can steer, see, and stop safely.

Introduction

Geometric design fixes the path of the roadway in plan (straights, curves, transitions) and profile (grades, vertical curves), together with lane width, crossfall, and clear zones. Stopping sight distance (SSD) combines reaction distance and braking distance; passing and decision sight distances are longer in many standards. Horizontal curves use radius and superelevation; intersections need turning paths and channelisation compatible with design vehicles.

Scatter plot of stopping distance versus speed with a fitted line
Stopping distance vs speed (illustrative data plot). See licence on Wikimedia Commons
Schematic of passenger car turning radii wall-to-wall and curb-to-curb
Passenger car turning circle (wall-to-wall and curb-to-curb). See licence on Wikimedia Commons

What geometric design fixes

  • Alignment — curvature, transition lengths, and design speed consistency.
  • Vertical geometry — grades, crest and sag curves for sight and drainage.
  • Cross-section — lanes, shoulders, medians, slopes, and clear width for recovery.
  • Interfaces — intersections, ramps, and terminal layouts for design vehicles.

Standards and guidance (South Africa)

National roads: SANRAL TRH series for geometric layout and sight distance. Other roads: COLTO and provincial or municipal manuals. Coordinate with traffic engineering (volumes, LOS) and safety audits; sight distance requirements often govern crest curves and barrier offsets.

SSD ≈ v t + v²/(2a)   with v = V/3.6 m/s when V is in km/h (level road, illustrative).

Notation (common)

  • V — design or operating speed (km/h in the calculator).
  • t — perception–reaction time (s).
  • a — longitudinal deceleration (m/s²).
  • R — curve radius; e — superelevation (in detailed design).

Examples in practice

  • Rural highway: SSD and passing sight on straights and crest curves.
  • Urban arterial: lane width, median openings, and intersection corner radii for buses and trucks.
  • Roundabout: entry deflection and inscribed circle for design vehicle swept path.
  • Mountain pass: limiting gradient and emergency stopping sight past tight curves.

Calculator — stopping sight distance

Enter V (km/h), t (s), and a (m/s²). Speed is converted with v = V/3.6 m/s; output is approximate SSD in metres (level road, no grade correction).

SSD = v t + v²/(2a)   (v = V/3.6 m/s)

SSD

Stopping sight distance SSD = vt + v²/(2a) from design speed, reaction time, and deceleration (level road).

Key terms

Sight distance
Distance visible ahead for stopping, passing, or decision—must exceed required values for design speed and context.
Design vehicle
Vehicle dimensions used to check lane width, turning radii, and swept paths.

Software and geometric design

Road projects combine alignment, profiles, cross-sections, and corridor models. Examples (official sites):

  • Autodesk Civil 3D — alignments, profiles, corridors, and superelevation.
  • OpenRoads Designer — road and corridor design (Bentley).
  • ArcGIS Pro — GIS context, terrain, and preliminary corridor siting.
  • PTV Visum — transport planning networks (links demand to geometric options).

No product endorsement—confirm design inputs, design vehicles, and sight checks against the governing standard and independent checks where required.

Diagram sources

Educational figures. Files in Images/geometric-design/ were downloaded from Wikimedia Commons into this repo (not copied from other topic folders). Confirm licence on each Commons file page before reuse.