Headway refers to the amount of time that passes between successive buses or rail vehicles. Service span refers to the amount of time between the time when the first bus of the day arrives to the time when the last bus of the day leaves. While both are important to a successful transit system, to the transit dependent a good service span is critical while a good headway is merely nice to have. The difference between waiting 60 minutes because you just missed a bus to waiting eight hours because you just missed the last bus is extreme, as all of you who live in an area served by a transit system that hurries home right after the sun sets can surely attest to. But to a certain extent we can have both - by expanding the headway, we can extend the service span at little to no cost, as this article describes . As the saying goes, we can have our bus and eat it too.
In the picture: San Bernardino CA's Omnitrans Route 61 lays over at Pomona TransCenter. Despite operating every 15 minutes on Sunday, Route 61 goes to bed at 6:28 PM.