Maybe GDP is a bad metric. In any other field, a metric this naïve would be laughed out the door.
If GDP is trying to capture net effective productivity, then let's stop using a metric that's 2 degrees of aggregated abstraction away from this metric of interest.
Cars are middlemen. They allow labor to reach a location that performs a productive activity. They produce no productivity of their own. IE. Cars are network IO. Whichever transport method maximizes throughput and minimizes latency for the least cost is ideal.
Any decent engineer knows that the key to better network IO is to:
* improve locality - (keep things near each other)
* batch things together - (buses and trains)
* reduce size of individual packets - (cycles instead of huge cars)
* no lossy transmission - (maintain worker productivity, stave off obesity)
* avoid waits as much as possible, let actions happen async (https://www.youtube.com/watch?v=pqQSwQLDIK8)