Spur route

A spur route is a usually short road forming a branch from a longer, more important route (typically a major road, freeway, Interstate Highway, or motorway). A bypass or beltway is never considered a true spur route as it typically reconnects with another or the same major road. Both loops and spurs can be forms of a special route.

In the province of Ontario, most spur routes are designated as A or B, such as Highway 17A, or 7B. A stands for "Alternate Route", and usually links a highway to a town's central core or main attraction, while B stands for "Business Route" or "Bypass", but are used when a main highway is routed around a town and away from its former alignment. The designation of "C" was used twice (Highway 3C and 40C), and is assumed to mean "Connector". Both highways have long since been retired and are now county roads. There was also one road with the D designation (Highway 8D, later the original Highway 102), and this may have stood for "Diversion", as it was along the first completed divided highway in Canada at the time (Cootes Drive in Hamilton).

In the US, many Interstate Highways have spur routes when they enter a large metropolitan area. Interstate spur routes are numbered with a three-digit number. The last two digits of the number are the number of the "parent" Interstate; e.g. a spur route of Interstate 90 could be 990; a spur route of Interstate 5 could be 105.

Spur Interstate routes have three-digit numbers with an odd first digit. A subsidiary route either passing through a city or bypassing it and then reconnecting to a major highway would receive an even first digit, and be considered a loop rather than a spur. For example, in the case of Interstate 5, Interstate 105 is a spur route ending at Los Angeles International Airport, whereas Interstate 405 begins and ends at Interstate 5, bypassing downtown Los Angeles.

...
Wikipedia