Intersections are where most conflict points, delay, and crashes concentrate on a road network. Design follows Austroads guidance, balancing capacity, safety, and cost across several control types.
Intersection Control Types
| Control Type | Typical Application |
|---|---|
| Priority (give way/stop) | Low-volume minor road intersections |
| Roundabout | Moderate volumes, balanced flows across approaches |
| Signalised | High volumes, pedestrian priority, or complex movements |
| Grade-separated (interchange) | Freeways, very high-volume arterials |
Roundabout Design
Geometric Elements
| Element | Function |
|---|---|
| Central island | Deflects traffic, forces speed reduction |
| Circulating carriageway | Width sized for design vehicle swept path |
| Splitter islands | Separate entry/exit, provide pedestrian refuge |
| Entry/exit radii | Control entry speed: smaller radii reduce speed but reduce capacity |
| Deflection | Horizontal deflection through the roundabout to control speed |
Roundabout Capacity
Entry capacity is primarily a function of the circulating flow opposing each entry (gap acceptance model):
$$ Q_e = A - B \cdot Q_c $$Where $Q_e$ = entry capacity, $Q_c$ = circulating flow, $A$, $B$ = empirical coefficients (geometry-dependent).
Roundabouts perform best with balanced flows across all approaches; a single dominant approach can create disproportionate delay on minor approaches.
Signalised Intersection Design
Signal Phasing
Phases group non-conflicting movements to run simultaneously, cycling through all approaches:
| Phase Type | Description |
|---|---|
| Leading/lagging turn phase | Protects a turning movement before/after the through phase |
| Split phasing | Each approach runs separately (used where opposing movements can't share safely) |
| Pedestrian phase | Dedicated or concurrent with a vehicle phase |
Cycle Time and Capacity
$$ C_o = \frac{1.5L + 5}{1 - \sum y_i} $$Where $C_o$ = optimum cycle time, $L$ = total lost time per cycle, $y_i$ = flow ratio (demand/saturation flow) for the critical movement in each phase.
Degree of saturation:
$$ x = \frac{q}{s \cdot (g/C)} $$Where $q$ = arrival flow, $s$ = saturation flow, $g/C$ = effective green ratio. $x > 1.0$ indicates oversaturation.
Design Vehicles and Swept Paths
Intersection geometry (corner radii, lane widths, roundabout circulating width) must accommodate the design vehicle for the location:
| Design Vehicle | Typical Application |
|---|---|
| Passenger car | Local residential intersections |
| Service vehicle (SV) | Collector roads, minor commercial access |
| Medium/heavy rigid truck | Arterial roads, industrial access |
| Articulated vehicle / B-double | Freight routes, industrial estates |
Swept path templates verify the design vehicle can complete the required turn without encroaching on kerbs, islands, or opposing lanes.
Pedestrian and Cyclist Provision
| Treatment | Application |
|---|---|
| Marked pedestrian crossing (zebra) | Low-speed, low-volume roads |
| Signalised pedestrian crossing | Higher-volume/higher-speed roads, part of signal phasing |
| Cyclist-specific signal phase | High cyclist volume corridors |
| Raised/refuge islands at roundabouts | Two-stage pedestrian crossing |
Practical Notes
- Roundabouts generally outperform signals for safety (fewer severe crash types) at moderate-volume, balanced-flow locations, but signals handle high, unbalanced, or pedestrian-heavy demand better.
- Entry deflection is the primary speed control mechanism at a roundabout: a roundabout with poor deflection behaves like an uncontrolled intersection at speed.
- Swept path checks should use the actual worst-case design vehicle expected at the site (e.g. articulated garbage trucks on residential collector roads), not just a generic passenger car template.