Princes Motorway is a The section between Bulli Tops and Gwynneville was known as Mount Ousley Road, and was first built as a defence route and later upgraded to dual carriageway standards.
It is the backbone of road traffic in the Illawarra. As Wollongong and Port Kembla are important industrial centres, freight traffic is heavy. Despite the current decline of the local steel industry, emergence of Wollongong as a commuter city of Sydney has kept the motorway busy.
Route
thumb|The northern terminus of Princes Motorway, looking south from , pictured in 2007.
In the north, Princes Motorway commences at the interchange with Princes Highway at Waterfall in Sydney and heads south as a four-lane, dual-carriageway road, taking more or less a parallel route with Princes Highway until the sprawling interchange with Appin Road and Princes Highway at Bulli Tops. It continues downhill, avoiding the steep Bulli Pass, and bypasses Wollongong CBD, through Gwynneville and continues for , bypassing the suburbs of Yallah and Albion Park Rail, reaching the interchange with Illawarra Highway (Terry Street) at Albion Park, before terminating with the existing alignment of Princes Highway at an interchange in Oak Flats. At 22.9km, it was then the longest section of freeway to completed at one time, at a cost of $30.5 million; This part of the freeway did not feature the Helensburgh Interchange (which subsequently opened in February 2000). The toll operated for 20 years: this was 10 years short of its intended operating length, due to local residents complaining that the F3 Freeway had its toll dropped in 1988 (which was at the time intended to be dropped as its loans had been fully paid off, unlike those of the F6). After much pressure, the tolls were eventually removed on 30 July 1995, when the loans had been repaid. Remnants of the tollbooths could initially be seen at the old toll plaza at Waterfall, such as faint markings and a set of warning lights in the southbound direction for the toll plaza. These remnants have since been removed. However, , the widened carriageways for the toll booths can still be seen at .
To complement the tollway, the dual carriageways of Princes Highway from Waterfall north to Loftus and the Sutherland bypass were constructed and opened to traffic on 16 September 1975. and is still often referred to as such. Mount Ousley Road was built in 1942 as a defence route, involving the reconstruction of part of a 19th century route from Bulli Tops to the Picton-Mt Keira road (the southern section not incorporated into the defence route is Clive Bissell Drive), and the construction of a new section of road to descend the escarpment and terminate at Princes Highway at North Wollongong (the easternmost 3.5 km of Picton Road, from Mount Keira Road to Mount Ousley Road, was also constructed as part of this project).
From the 1960s to the 1980s Mount Ousley Road was gradually upgraded,
In November 2015, it was announced that the section between Bulli Tops and Picton Road would have a third lane added in each direction. , detailed design works have been completed.
Southern section (Gwynneville to Yallah)
The construction of the first stage of Princes Motorway between Gwynneville and Yallah commenced in May 1959. This formed the majority of what was built as a north-south bypass of Wollongong central business district, through the Parliament of New South Wales provided for the declaration of Main Roads, roads partially funded by the State government through the Main Roads Board (MRB). With the subsequent passing of the Main Roads (Amendment) Act of 1929 to provide for additional declarations of State Highways and Trunk Roads, the Department of Main Roads (having succeeded the MRB in 1932) declared Southern Freeway as a motorway (under plan number 6006), on 8 October 1975,
Freeway Route F6 was allocated to the southern section of Southern Freeway in 1973, and along the entire northern section when it opened in 1975: as new sections of the freeway opened, Freeway Route F6 was extended along these new sections, but had already begun to be phased out in the mid-1908s to be replaced by National Route 1, and had disappeared by 1992; the Mount Ousley Road section was designated part of National Route 1 from 1975. With the conversion to the newer alphanumeric system in 2013, National Route 1 was replaced with route M1, and Southern Freeway and Mount Ousley Road were officially renamed as M1 Princes Motorway.
Albion Park Rail Bypass
At the southern end, Princes Motorway was extended to Oak Flats via a 9.8 km bypass of Albion Park Rail. The bypass completed the 'missing link' in the four-lane road between Sydney and Berry (since extended to Bomaderry/Nowra), The section of the bypass between Yallah and the Illawarra Highway (Terry Street) interchange was opened to traffic in May 2021. The northbound carriageway of the remainder of the bypass (ie north from the New Lake Entrance Road interchange to the Illawarra Highway interchange) opened to traffic on 7 August 2021. The remainder of the southbound carriageway was opened to traffic on 9 October 2021, thereby completing the bypass.