The town of Busia was once the primary border crossing between Kenya and Uganda, with construction on the Mackinnon-Sclater Road commencing as early as 1890. This road, designed for oxcart travel, connected Mombasa to the Busia border crossing. However, its significance waned with the construction of the Uganda Railway, leading to a shift in the main border crossing to Malaba, located slightly north. During the British colonial era, a network of roads was established, with the route via Kisumu being a notable one, although the more northerly route via Eldoret received greater attention. In recent years, significant upgrades have been made, particularly in Kisumu. A grade-separated bypass was constructed between 2013 and 2014, improving traffic flow. Additionally, a trumpet interchange connecting to the A8 was completed at Mau Summit in 2017, further enhancing the road network's efficiency.[2]
The road was initially designated as the A109, a prominent route within the East African numbering system that spanned three countries. However, it was later reclassified as the B1, indicating a lower hierarchical status. This reclassification occurred during a period when the A104 motorway to the north was considered the primary connection. In 2016, the road underwent another renumbering, this time being elevated to the A12 designation. This change reflects a reevaluation of the road's importance within the regional transportation network.[2]
