|Riddoch Highway near Padthaway|
|Length||239 km (149 mi)|
|North end||Dukes Highway, Keith, South Australia|
|South end||Cape Northumberland, Port MacDonnell, South Australia|
|Major settlements||Padthaway, Naracoorte, Coonawarra, Penola, Tarpeena, Mount Gambier|
The Riddoch Highway in south-eastern South Australia branches from the Dukes Highway at Keith and travels south through Padthaway, Naracoorte, Penola, Nangwarry, Tarpeena, and Mount Gambier to Port MacDonnell and nearby Cape Northumberland.
The highway has been designated as State Route A66 between Keith and Mount Gambier. The remainder, between Mount Gambier and Port MacDonnell has been labelled State Route B66, although this is not officially known as the Riddoch Highway.
It passes through grazing and cereal-growing land, horticultural and vineyards (within the following wine regions - Padthaway, Wrattonbully, Coonawarra and Mount Gambier), and plantation timber, predominantly pinus radiata.
|Tatiara||Keith||0.0||0.0||Dukes Highway (National A8) – Adelaide, Melbourne|
|Naracoorte Lucindale||The Gap, Cadgee, Lochaber, Keppoch||83.5||51.9||Bordertown-Naracoorte Road|
|Naracoorte||109||68||Wimmera Highway (C240) –Hynam, Edenhope|
|Wattle Range||Glenroy||141||88||Edenhope-Penola Road (C212) – Edenhope, Victoria|
|Wattle Range||Penola||159||99||Casterton Road (C198) – Casterton, Victoria, Melbourne|
|Mount Gambier||Mount Gambier||210||130||/ Princes Highway – Portland, Victoria, Millicent, Kingston SE|
|Grant||Port MacDonnell||238||148||Cape Northumberland|
|1.000 mi = 1.609 km; 1.000 km = 0.621 mi|
- "Naming of State Rural Roads South East (RACK PLAN 994)" (PDF). Department of Planning, Transport and Infrastructure. Retrieved 18 November 2014.
- Longbottom, Mardi; Maschmedt, David and Pichler, Markus (2011), Unearthing viticulture in the Limestone Coast (PDF), p. 12, ISBN 978-0-646-56744-0, archived from the original (PDF) on 6 November 2014, retrieved 5 November 2014
- "roadwatch report – riddoch highway" (PDF). RAA. September 2006. Retrieved 2007-10-16.
- Google (6 February 2018). "Riddoch Highway" (Map). Google Maps. Google. Retrieved 6 February 2018.
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