Statistics:
- Length:
- 154 km
- Termini:
- Northern: Armadale Rd (SR14) and Albany Hwy (SR30), Armadale
- Southern: Robertson Dr (NR1 / SR10) and Picton Rd (unnumbered), Glen Iris
- Miscellaneous:
- Continues as NR1 from Bunbury
- Suburbs, Towns & Localities Along Route:
- Armadale, Mount Richon, Wungong, Darling Downs, Byford, Cardup, Whitby, Mundijong, Jarrahdale, Mardella, Serpentine, Keysbrook, North Dandalup, Fairbridge, Pinjarra, Blythewood, Coolup, Waroona, Hamel, Wagerup, Yarloop, Cookernup, Warawarrup, Harvey, Wokalup, Benger, Brunswick Junction, Roelands, Waterloo, Picton East, Picton and Glen Iris (Bunbury)
Route Numbering:
- Current: 20
- Multiplexed with: 22 206
- Former: 1
- Road Authority Internal Classification: 1
- H009
General Information:
State Route 20 is a north-south route connecting the southwestern suburbs of Perth with Western Australia's third largest city, Bunbury.
Once the primary thoroughfare connecting Perth to the southwest, this route links a multitude of small towns that owe their existence to the 19th-century construction of the railway line that closely parallels much of State Route 20's length.
The route is rural highway standard for most of it, and services residential, commercial, light industrial and agricultural areas.
Multiplexes along the route include:
- State Route 22, for 400 m, Jarrahdale.
- Tourist Drive 206, for 11 km, between Jarrahdale and Keysbrook.
History:
- 1 July 1853: Colonial Secretary Frederick Barlee announced a new proposal for a Perth–Pinjarra–Bunbury route along the foothills of the Darling Scarp. 2
- 1864: Work commences on construction of the route announced in 1853 by two gangs of convicts.
- 1876: Construction completed of the route announced in 1853.
- July 1941: The name South Western Highway officially adopted. 3
- 1980s: National Route 1 rerouted via Old Coast Road, SR20 introduced to the former NR1 alignment between Armadale and Bunbury.
- 2006: Additional passing lanes added to the highway, one south of Yarloop, one north of Harvey, and two north of Brunswick Junction. 4
- April 2022: Work commenced on construction of the interchange with Bunbury Outer Ring Road (BORR) at Waterloo, near the roadhouse. 5
- 2024: State Route 107 decommissioned between Bunbury and Roelands, extinguishing the duplex with State Route 20. 6
1 WA Government, Main Roads Western Australia, Road Information Mapping System.
2 NK Perry, History and Present Potentialities of the Old Coast Road from Mandurah to Bunbury.
3 South Western Advertiser, Armadale-Kelmscott Road Board, 25 July 1941.
4 WA Government, Department of Infrastructure and Planning.
5 WA Government, Main Roads Western Australia, Projects & Initiatives, Bunbury Outer Ring Road, Previous Publications, Construction Update, April 2022.
6 WA Government, Main Roads Western Australia, Western Australian State Routes 2024.