History of King's Highway 3A (Niagara):
Highway 3A was an alternate route to Highway 3 that was established to serve the new Peace Bridge when it opened to traffic in 1927 between Fort Erie and Buffalo, New York. Provincial Highway 3A was first assumed by the Department of Public Highways (DPHO) on May 11, 1927. Prior to 1927, there was no provincial highway route through the southern portion of Welland County. In addition to serving the Peace Bridge at Fort Erie, Highway 3A also provided improved highway connections to Port Colborne. Highway 3A branched off of Highway 3 at Chambers Corners and continued for 43 km to the foot of the Peace Bridge in Fort Erie. Initially, Highway 3A was gravel-surfaced for its entire length, although it did not remain that way for very long. The entire highway was paved from Chambers Corners to Fort Erie in 1928.
The original route of Highway 3A through Port Colborne followed Killaly Street. However, on May 2, 1928, the DPHO assumed a new route for Highway 3A through Humberstone. This new highway diversion through Humberstone provided an alternate route for traffic crossing the Welland Canal, while the canal was being diverted and reconstructed. The Humberstone Diversion was paved in 1929. It is believed that the Humberstone Diversion was also signed as Highway 3A. On June 15, 1930, the original alignment of Highway 3A via Killaly Street was decommissioned, once the New Welland Canal Bridge at Humberstone opened. That same year, Provincial Highway 3A was re-designated as King's Highway 3A.
In order to improve the highway numbering logic in the Niagara Falls area, the Highway 3 and Highway 3A designations in Welland County were reversed in 1930 . Previously, Highway 3A ran from Chambers Corners to Fort Erie, via Port Colborne, while Highway 3 ran from Chambers Corners to Niagara Falls, via Welland. It was determined that the former route of Highway 3A provided a much more direct highway connection to the United States than the route offered by Highway 3. As a result, the route designations were swapped. The old route of Highway 3 to Niagara Falls via Welland became Highway 3A. The new 419 km route of Highway 3 to Fort Erie created a more convenient through route for travelers heading between the Peace Bridge in Fort Erie and the Ambassador Bridge in Windsor. The post-1931 length of Highway 3A was 40 km.
In 1937, Highway 3A was designated concurrently with Highway 58 between Welland and the Highway 20 Junction near Allanburg. A short diversion for Highway 3A and Highway 58 was completed at the north side of Welland in 1937, which saw Merrittville Highway realigned so that it formed a continuous through route with North Main Street (now known as Niagara Street). The new diversion, about a half-mile in length, was designated as a King's Highway on November 30, 1937. The old Highway 3A and Highway 58 alignment, via Aqueduct Street, was later transferred back to the City of Welland. In 1959, the Welland Bypass was completed and opened to traffic. This new arterial road followed the route of the Niagara-St. Catharines and Toronto Electric Railway from Thorold Road to West Side Road, forming a new western road link which bypassed Downtown Welland. The new route was called Prince Charles Drive, and was designated as Highway 3A & Highway 58 on May 28, 1959. Highway 3A & Highway 58 was also designated along Thorold Road between Prince Charles Drive and Merrittville Highway (Niagara Street).
In order to simplify the highway numbering between Welland and Niagara Falls, Highway 3A was truncated at the Highway 58 Junction in Welland in 1965. The remainder of Highway 3A's route from Welland to Niagara Falls ran concurrently with either Highway 58 or Highway 20. It was decided that the concurrent designation between Highway 3A and these two routes was no longer necessary. The removal of the Highway 3A designation between Welland and Niagara Falls resulted in a reduction of Highway 3A's length to only 15.6 km. The last surviving section of Highway 3A between Welland and Chambers Corners was formally decommissioned in 1970, finally erasing this once important highway from existence. The route of Highway 3A was transferred to the newly-formed Regional Municipality of Niagara on August 20, 1970. The former route of Highway 3A is now known as Niagara Road 24 & Niagara Road 27. A different, unrelated Highway 3A existed in Windsor during the late 1920s and early 1930s.