During 19th century The first recorded major improvement to the route of Great Western Highway was the construction in 1806 of ten bridges along Parramatta Road. In attempts to improve the gradient of the descent from the Blue Mountains plateau to the floor of the Hartley Valley, Lawson's Long Alley was opened in 1824. This still did not prove satisfactory and construction of a second deviation, known as Lockyer's Pass, was commenced. However this route was not completed, as its construction was abandoned in favour of construction of Mitchell's route via Victoria Pass. Originally the Great Western Road crossed the Nepean River at Penrith by means of a ferry adjacent to the Log Cabin Hotel. This was superseded in 1856 by a bridge which was destroyed by a flood in 1857. A second bridge was opened in 1860, and was destroyed by the record flood of 1867. In the same year a new bridge,
Victoria Bridge, was nearing completion adjacent to the road crossing, as part of the Penrith-Weatherboard (Wentworth Falls) section of the Main Western Railway. Its deck was modified to accommodate road traffic as well as the single-track railway. This bridge continued in dual use until 1907 when the current steel truss railway bridge was built alongside, and the 1867 bridge was given over solely to road traffic. This bridge remains in use for Great Western Highway. The design of this bridge is almost identical to that of the 1863
Menangle Railway Bridge, also over the Nepean River. In 1870 the
Denison Bridge, a wrought iron
truss bridge, was built across the Macquarie River at Bathurst. This replaced an earlier wooden bridge.
During early 20th century In the first few years of the 20th century the railway overpasses between Lapstone Hill and Mount Victoria were replaced as part of the duplication of the Main Western railway. These bridges were of brick arch construction. They were in turn replaced in the mid-1950s to obtain the necessary height clearances for overhead wiring for the electrification of the Main Western Railway from Penrith to Bowenfels. In 1912 Victoria Pass was superseded by Berghofers Pass, which followed a similar route to Victoria Pass, but below it. It was more winding and thus longer, thereby affording a less steep climb. However rapid improvements in motor vehicle performance meant that in 1920 Victoria Pass was rebuilt to become the main route again. After the ascent of the eastern escarpment by the Main Western Railway was deviated for the second time in 1913 to its current route via Glenbrook Gorge, the Great Western Road was also deviated at this point for a second time in 1926 by the then Main Roads Board, which rerouted it via the disused
1867 stone arch railway viaduct across Knapsack Gully and around the southern side of Lapstone Hill to gain the first plateau in the ascent of the Blue Mountains. As this viaduct had held only a single railway track, its deck was widened in 1939 to its present two lane configuration. The viaduct was closed to motor traffic when the M4 Motorway was extended west from Russell Street to connect to the highway at Lapstone in 1994. West of Knapsack Gully, although now widened to four lanes, the 1926 route of the highway is still in use. It uses a long stretch of abandoned railway
formation – the section from Zig Zag Street to Blaxland station is located on the original 1867 railway alignment. An indication of the need to divert the railway can be gained from the gradient of the highway as it climbs west from Hare Street to Lovett Street. A number of deviations were built in 1929: • at Haslams Creek, , to straighten the road and provide a new bridge • between and , to remove an underpass and an overpass of the railway from the route of the highway • at Eusdale, Yetholme (in three sections) and Melrose (all west of Meadow Flat), to ease gradients. In 1930 the level crossing at Bowenfels was replaced by an underpass. Victoria Pass was upgraded in 1932 to give a constant width of 8.5 m, with a minor deviation built at the foot of the pass.
Upgrading since World War II In 1957 a short deviation immediately west of Linden eliminated two narrow overpasses of the railway, both of which had right angle bend approaches from both directions. These bridges would have had to have been replaced in any event to allow for overhead wiring for the electrification to Bowenfels of the Main Western Railway. Such replacements occurred east of Linden, further west of Linden near Bull's Camp, east of Lawson, at Medlow Bath and east of Mount Victoria, and in these cases the replacement bridges were located at a skew angle to eliminate right angle bend approaches, with the earlier bridges left for pedestrian use. In 1958 the 'Forty Bends’, where the Highway runs along the foot of Hassans Walls approaching Lithgow, were eased. The fact that this section of the highway is on the southern side of a very high escarpment poses severe ice problems during winter, due to the lack of sunlight. In the early 1960s the highway was deviated east of Leura to cross under the railway at Scott Parade (which was itself part of the former highway route) and run along the north side of the railway to rejoin the previous route at Leura Mall. In 1967, the highway was deviated to bypass Springwood shopping centre, eliminating two narrow underpasses of the railway from the highway route. The previous route remain in use for local traffic as Macquarie Road. In 1968 a dual carriageway 3 km deviation was opened at Prospect. This replaced the only winding section of the Highway between Parramatta and Penrith. The
bypassed section was heritage-listed in 2014 as the best-preserved section of the 1818 alignment. From the late 1960s to the early 1970s the highway was almost entirely realigned and constructed to three lanes, being deviated as necessary, between Kirkconnell and Glanmire. Ironically this included reinstatement of most of the parts of Major Mitchell's 1830 alignment which had been deviated in 1929 to ease gradients. This work was extended eastward to Mount Lambie in stages during the late 1980s and early 1990s. A second Cox's River deviation, to replace the 1929 deviation, was completed in 1993, between Marrangaroo and Mount Lambie. At Katoomba the highway was deviated in 1985 to travel along the eastern side of the
railway station, whereas the original alignment crossed the railway via a level crossing at the north end of Katoomba Street and ran along the western side of the railway. Immediately west of where the highway now crosses the railway due to this deviation, the highway was realigned over a distance of 1 km in 2004 to remove the sharp bend at 'Shell Corner'. In Bathurst, the Denison Bridge (1870) was bypassed in 1991 by a realignment of the Highway where it crosses the Macquarie River into Bathurst city centre. Because of its heritage value it was retained for use by cyclists and pedestrians. During 1991–1993 a massive cutting was made to improve and widen the alignment of the highway immediately east of Woodford. At the top of the southern side of this cutting can be seen the rudimentary excavation of the rock for Cox's 1815 road. This was severed in 1868 by the construction of the Springwood-Mount Victoria section of the Main Western Railway. The railway itself was deviated at this point in the 1920s when it was duplicated, and a cutting on the original alignment of the railway now forms the top section of the southern face of the highway cutting, the terrace in the face of the cutting being the bed of the original cutting. In June 1993 the highway route was severed at
Emu Plains with the closure to road traffic of the
Knapsack Gully Viaduct. This occurred in conjunction with the westward extension of the M4 Motorway from its terminus since 1971 at Russell Street, Emu Plains. This extension connects directly to the highway at
Lapstone, bypassing the viaduct. The portion of Great Western Highway west from Russell Street to Mitchells Pass Road is now only used by local traffic to access residential properties. Mitchells Pass, travelling is now one way eastbound between Lennox Bridge and the highway, due to its narrowness. Great Western Highway traffic therefore has to use the M4 between Russell St and Governors Drive. A major realignment west from
Mount Boyce (the highest point on the highway) to eliminate the Soldiers Pinch and other nearby sharp curves was completed in 2002.
Duplication and widening From Railway Square to Woodville Road, the highway was widened to its present width when it was reconstructed in reinforced concrete in the 1930s. From Woodville Road west to The Northern Road the highway was widened, generally progressively westward, from the mid-1960s to the late 1970s. This section is a combination of six lanes with median strip, six lanes with wide landscaped median, and four lanes undivided. At The Northern Road in Kingswood the highway reverts to a four lane undivided configuration through Penrith shopping centre, widens to six lanes at the Castlereagh Road intersection, reverts to two lanes west from Castlereagh Road to Russell Street, and is then four lanes undivided with sealed shoulders from Russell Street to the base of Mitchells Pass, where it has been truncated. Between 1981 and 2015 the
NSW Government Roads & Maritime Services duplicated the highway between and west of Katoomba, generally working westward. This work incorporated the dual carriageway Springwood bypass, opened in 1967. West of Katoomba, there is a further of dual carriageway on the River Lett Hill, and of dual carriageway from Magpie Hollow Road at Old Bowenfels to Lidsdale State Forest. This was completed as follows: • road widening and realignment completed in 2006 • River Lett Hill 2014/15 • Old Bowenfels to Bowenfels completed in sections during the late 1990s • Bowenfels to Marrangaroo Creek mid-1970s • Marrangaroo interchange with the Castlereagh Highway 1970 • Marrangaroo interchange to Lidsdale State Forest (second Cox's River deviation) 1993. Four-lane undivided sections are the of Victoria Pass (except for Mitchell's 1832 causeway, which is two lanes), and from Boyd Street in to the terminus of the highway at its junction with the Mitchell and Mid Western Highways in West Bathurst (parts have a raised median strip). From Lidsdale State Forest as far west as Glanmire most of the highway is three lanes, with almost continuous overtaking lanes alternating between eastbound and westbound.
Controversy Many people, including Aboriginal communities, are complaining about the expansion works. The completed highway will destroy a lot of native lands. There are also fears that the highway could wipe out platypus populations. ==Current projects==