The Locomotive Acts (or Red Flag Acts) were a series of Acts of Parliament in the United Kingdom regulating the use of mechanically propelled vehicles on British public highways during the latter part of the 19th century.
The first three, the Locomotive Act 1861 (24 & 25 Vict. c. 70), the Locomotives Act 1865 (28 & 29 Vict. c. 83) and Highways and Locomotives (Amendment) Act 1878 (41 & 42 Vict. c. 77), contained restrictive measures on the manning and speed of operation of road vehicles; they also formalised many important road concepts such as vehicle registration, registration plates, speed limits, maximum vehicle weight over structures such as bridges, and the organisation of highway authorities.
The most strict restrictions and speed limits were imposed by the 1865 act (the "Red Flag Act"), which required all road locomotives, which included automobiles, to travel at a maximum of 4 mph (6.4 km/h) in the country and 2 mph (3.2 km/h) in the city, as well as requiring a man carrying a red flag to walk in front of road vehicles hauling multiple wagons.
The 1896 act removed some restrictions of the 1865 act and raised the speed to 14 mph (23 km/h).
The Locomotives on Highways Act 1896 provided legislation that allowed the automotive industry in the United Kingdom to develop soon after the development of the first practical automobile (see History of the automobile). The last "locomotive act" was the Locomotives Act 1898.