NYC subway weekday on-time performance, measured as the "percentage of trains that arrive at the terminal within 5 minutes of
the scheduled arrival time." Source: mta.info.

While we appear to be hurtling toward a future of less reliable transit service, at least those of us with cell phones will be able to plan accordingly:

The Metropolitan Transportation Authority (MTA) today launched an email and text messaging system that will notify registered customers of planned and unplanned service changes at any of the MTA’s family of transportation agencies… The system will be fully operational tomorrow morning.

Using the MTA’s website at www.mta.info, customers can register to receive alerts about any combination of subway lines, bus routes, rail lines, bridges or tunnels. They can choose to receive them 24/7, or only during a particular time of day or week.

Eat your heart out, Twitter.