M31 may refer to:
The Metropolitan Transportation Authority (MTA) operates a number of bus routes in Manhattan, New York, United States. Many of them are the direct descendants of streetcar lines (see list of streetcar lines in Manhattan).
Presently, the New York City Transit Authority and its subsidiary Manhattan and Bronx Surface Transit Operating Authority operate most local buses in Manhattan. The Roosevelt Island Operating Corporation operates the Roosevelt Island Red Bus Service on Roosevelt Island.
The first bus company in Manhattan was the Fifth Avenue Coach Company, which began operating the Fifth Avenue Line (now the M1 route) in 1886. When New York Railways began abandoning several streetcar lines in 1919, the replacement bus routes (including the current M21 and M22 routes) were picked up by the New York City Department of Plant and Structures (DP&S). The DP&S began operating several other buses (including the current M79 and M96 routes) in 1921. All of these but the M21 were acquired by Green Bus Lines in 1933; Green transferred several of these to the Comprehensive Omnibus Corporation in 1935.
The M31 was planned as a Reading to M3 motorway which was dubbed the 'M3 – M4 link motorway'. It would have provided a direct high-speed route between the two motorways. The motorway was planned at the same time as the largely unrealised London Ringways scheme and an additional section was planned that would have taken the M31 south and east from the M3 to connect to the scheme's Ringway 4 (now the M25) providing a shorter route for traffic travelling between the west and Surrey and Kent.
Only part of the planned route was built; it bears the designations A329(M) and A3290. Had the motorway been constructed it would have provided an alternative to the south-west section of the M25, reducing the motorway journey between Reading and Byfleet by approximately 10 miles (16 km) and alleviating traffic on what is the M25's busiest section.