Geometric design determines the physical layout of a road: its alignment, curvature, gradients, and cross-section, to provide safe, efficient travel at a chosen design speed. In Australia, geometric design follows the Austroads Guide to Road Design.
Design Speed
Design speed is the key parameter driving nearly every other geometric element (curve radii, sight distance, superelevation).
| Road Type | Typical Design Speed |
|---|---|
| Local access street | 40–50 km/h |
| Collector road | 60–70 km/h |
| Arterial road | 70–90 km/h |
| Rural highway | 100–110 km/h |
| Freeway | 110–120 km/h |
Sight Distance
Stopping Sight Distance (SSD)
The minimum distance a driver needs to see and stop safely for an obstacle:
$$ SSD = 0.278 V t + \frac{V^2}{254(f \pm g)} $$Where:
- $V$ = design speed (km/h)
- $t$ = reaction time (typically 2.0–2.5 s)
- $f$ = coefficient of friction (speed-dependent, ~0.28–0.40)
- $g$ = grade (as a decimal, + for uphill in the direction of travel)
Overtaking Sight Distance (OSD)
Required on two-lane, two-way roads to permit safe overtaking manoeuvres: significantly longer than SSD (often 400–800+ m depending on speed).
Horizontal Alignment
Minimum Horizontal Curve Radius
$$ R_{min} = \frac{V^2}{127(e + f)} $$Where $e$ = superelevation (as a decimal), $f$ = side friction factor.
| Design Speed (km/h) | Typical Min. Radius (m), $e_{max}=6\%$ |
|---|---|
| 60 | ~150 |
| 80 | ~280 |
| 100 | ~470 |
| 110 | ~600 |
Superelevation
Superelevation (cross-fall toward the inside of a curve) counteracts lateral (centripetal) acceleration:
$$ e + f = \frac{V^2}{127R} $$Maximum superelevation is typically limited to 6–7% for general roads (lower in urban areas, higher for high-speed rural roads/freeways).
Transition Curves
Spiral transition curves ease the rate of change of curvature (and superelevation) between straights and circular curves, improving comfort and reducing the perception of "curve shock."
Vertical Alignment
Grades
| Road Type | Maximum Grade (typical) |
|---|---|
| Freeway | 4% |
| Rural highway | 6–8% |
| Urban arterial | 8% |
| Local street | 12–15% |
Vertical Curves
Vertical curves (crest and sag) smooth the transition between grades, sized primarily to satisfy sight distance requirements:
Crest curve length (sight distance governed):
$$ L = \frac{A S^2}{100(\sqrt{2h_1} + \sqrt{2h_2})^2} \quad (S < L) $$Where $A$ = algebraic difference in grades (%), $S$ = sight distance, $h_1$, $h_2$ = driver eye height and object height.
Sag curve length (headlight sight distance governed):
$$ L = \frac{A S^2}{200(h + S\tan\beta)} \quad (S < L) $$Cross-Section Elements
| Element | Typical Dimension |
|---|---|
| Traffic lane width | 3.0–3.5 m |
| Shoulder width (sealed) | 0.5–2.5 m depending on road class |
| Median width (divided roads) | 1–6+ m |
| Verge/footpath | Per local urban design requirements |
Practical Notes
- Design speed should reflect the speed drivers will actually adopt based on the road's context (curvature, roadside development), not just the posted limit: a mismatch is a common crash contributor.
- Sight distance, not curve radius alone, is often the binding constraint on rural crest curves: check both together.
- Superelevation transition length must be coordinated with the horizontal transition curve length; treating them independently produces an uncomfortable or unsafe rate of rotation.