Mount Barker Road

A vestigial portion of Mount Barker Road exists through the "Old Toll Gate" at Glen Osmond, but has completely subsumed into the South Eastern Freeway as far as the Devil's Elbow, where a surviving western alignment of the road turns a sharp hairpin and has a winding uphill through Eagle On The Hill and rejoins South Eastern Freeway near Measday's Hill in Crafers West.[4]

The eastern portion of Mount Barker Road resumes from the freeway's Stirling exit, passing through the main shopping strips of Stirling and Aldgate[4] as State Route B33. From Aldgate, Route B33 continues on Strathalbyn Road, while Mount Barker Road follows the railway line as Tourist Route 57 to Bridgewater and under the freeway at Verdun where it joins State Route B34 (from Onkaparinga Valley Road) for a little over 3 kilometres (1.9 mi) through Hahndorf. At the south end of Hahndorf route B34 turns under the freeway towards Echunga and Mount Barker Road continues on the north side to the Mount Barker Interchange at Totness, where it changes name to Adelaide Road, and eventually ends at Mount Barker.[4]

Mount Barker Road in 1900

Mount Barker Road served as an early alignment of Princes Highway some time after it was first declared in South Australia in February 1922,[5] initially defined along Mount Barker Road and the route via now known as Strathalbyn Road from Adelaide via Aldgate, Mylor, Macclesfield, Strathalbyn, Langhorne Creek, crossing the Murray River at Wellington and then continuing along the present route beyond Meningie,[6] By 1928, the route was re-aligned to run further along Mount Barker Road through Mount Barker, and along Wellington Road via Wistow and Woodchester to Langhorne Creek,[7] although by 1935 this alignment was changed to run via Nairne, Kanmantoo, Murray Bridge and Tailem Bend (along what is now known as the Old Princes Highway).[8]

Princes Highway functioned as Adelaide's major south-eastern approach and was heavily trafficked; government plans to upgrade this route weaving through the Adelaide Hills had begun by the early 1960s. Initial sections of the road project that eventually became the South Eastern Freeway commenced in December 1965, along a 2 km section from Measday's Hill to Stirling: the eastbound carriageway was opened to traffic in 1967, and the section was fully completed in 1969 when the remaining carriageway and the Crafers Interchange became fully operational. The next stage extended the freeway east through Bridgewater to Verdun and opened in 1972, and from Verdun to Mount Barker in 1974, completely replacing Princes Highway - renamed back to Mount Barker Road - as the main route between Mount Barker and Crafers. As this was part of Highway 1, the route was progressively re-aligned along the freeway as sections of it opened. Mount Barker Road today remains as the main street of Stirling, continuing to Aldgate, Bridgewater, under the freeway at Verdun, then through Hahndorf to end near the Mount Barker interchange of the freeway. The road continues south over the interchange as Adelaide Road (reflecting the reverse journey) to terminate in Mount Barker.

The last part of the South Eastern Freeway to be built reconstructed the Adelaide end of Mount Barker Road including a grade-separated intersection at Mount Osmond Road. It replaced the infamous Devil's Elbow hairpin with a partial interchange. The new freeway passes through the Heysen Tunnels and rejoins the former alignment the other side of a steep ridge. Mount Barker Road itself climbs over and around the ridge. Since the four-lane dual carriageway is no longer required, part of the uphill side has been replaced with a bike path used as part of a popular cycling route to Mount Lofty and all motor traffic uses the former downhill carriageway. The section between the Measdays interchange and Crafers was also completely replaced by the freeway being built on the same alignment at a lower level in a deep cutting.[9]