As an engineer, I find it amazing that aside from cleaner, more efficient, quieter, and safer, we really haven't significantly improved mainstream jet travel speed for over 50 years. Supersonic must be a major barrier for all practical purposes.
The passenger miles per unit of fuel were not in favor of super sonic planes which were originally developed to compete with the smaller jets (707) that were around when development started. Those jumbos (747) made air travel really cheap.
By its predecessor Sud Aviation and BAC.
This video shows the difference pretty clearly: http://www.youtube.com/watch?v=1_ptUrQOMPs (it's a head-on crash test between a 1959 Bel Air and a 2009 Malibu).
Cars are immensely safer than the death-traps they were in the past. They're also immensely more comfortable, mind-bogglingly more fuel efficient, and produce a hell of a lot less pollution.
These aren't marginal improvements.