The slowest bus in New York City travels at an average speed of 4 mph at noon on a weekday -- not much faster than the average speed of a person walking.So says the Straphangers Campaign and Transportation Alternatives, a public transit advocacy group that has awarded the M23 in Manhattan a "Pokey" award for slowest bus route.The group also awarded the "Schleppie" to the city's least reliable bus: The M1, which runs across Fifth, Madison and Park avenues. The group says that one in three of the M1 route buses are off schedule or have big gaps in service.The Straphangers Campaign was founded in 1979 by the New York Public Interest Research Group.Transit officials have said they are working to bolster service through rapid-bus projects.