Its not just about NYC. The vacuum problem is a real issue for congestion. During peak hours there are plenty of people with cars that avoid the road due to tra

Its not just about NYC. The vacuum problem is a real issue for congestion. During peak hours there are plenty of people with cars that avoid the road due to traffic but would jump right in if the roads were less congested. So increased supply in road space (due to building new roads or this hypothetical carpooling scheme) is immediately met by additional driver demand and congestion quickly returns to where it was before.