Frequency and Duration *rule* in Mass Transit

TL;DR --> Frequency and service duration disproportionately effect the value of mass transit, and cuts in them have tremendous network effects making the transit much less valuable. 

Think of it this way, if you have to wait an hour for the next train, just take the car.
If you can't get back by train, just take the car.
Making it worse, if you make do during the week, but can't take the train on weekends, you'll definitely buy a car, and then you'll always have a reason to just take the car in the morning...

All this from  an excellent article by Jarret Walker

Frequency has three independent benefits for the customer, which helps to explain why high frequency is so critical to sustained high ridership:

• It reduces waiting, which is everyone’s least favorite part of a trip.  (No, a smartphone that tells you when the bus comes doesn’t solve the problem of waiting; we are still talking about time when you’re not where you want to be.)  The basic sensation of being able to go when you want to go is the essence of frequency.
• It makes connections easy, which makes it possible for a pile of transit lines become a network.  In transit, this is huge.  A transit line without good connections is useful for travelling in one dimension, along that line.  A network of frequent lines makes it easy to travel in two dimensions – all over the city, or at least all over the part of it that supports frequent service.   This network effectmassively expands the usefulness of every line in the network, thus increasing each line’s ridership potential. 
• Finally, frequency is a backstop for problems of reliability.  If a vehicle breaks down or is late, frequency means another will be along soon. 

If you think about how these three things govern the real usefulness of transit, you can begin to see why frequency is such a ferocious ridership-driver.  Notice that these three mechanisms are logically independent of each other, so they represent three differentways that a frequency change transforms the usefulness of transit for the better.  So it helps to think of frequency as cubed value; its benefits tend to be exponential, up to point, because improving frequency is actually three different improvements at once.

Another critical feature is Duration - as he points out

Finally, the duration of service matters, and it works much the way frequency does.  Service later into the evening, or on weekends, initially appears to be a bad investment, because we're adding lots of service when there aren’t as many riders.  But in the long run, its availability tends to correlate with high ridership.   That’s because riders won’t use the service in one direction unless they can get back, so evening service, even if the buses aren’t full, is a key part of how we build high ridership all day.   The same is true of weekends.  If you commute five days a week including some weekend days – like many people in the retail, entertainment, or service sectors – you are unlikely to rely on transit unless it works for you on all of those days.  One of the key features of ourHouston redesign is bringing weekend service up to the same level as weekdays, so that except for the weekday rush hour, the bus comes the same time every day.