Why Hasn't PRT Caught On?

Johnson says that PRT is "appealing and affordable". So why isn't PRT getting built?

Johnson says that early versions have been lacking, and that development can take a long time. But there are also a number of roadblocks on the outside:

"Policy and its feisty cousin, politics, are the biggest barriers (haven't we all noticed?) to doing anything new, not already proven. Transportation policy basically operates to protect existing systems - roads and mass transit. Protectors of both line up with legislators to stand guard at the gate, warding off any threat to the pool of ever-scarcer resources. Understandably, they want to add to what they have and hold on to what they've got."