Although 1 person driving another person from point A to point B reduces the number of single-occupancy vehicle trips, it doesn't necessarily reduce congestion. It could even increase congestion because all of those "private shuttles" are putting in so many miles on the roads when people would otherwise take higher-occupancy options or drive themselves (fewer miles, but that dreaded single-occupancy vehicle trip count goes up).
Well you're in luck, because that's why traffic engineers measure vehicle-miles-traveled and private shuttles saved almost 29 million miles.
On a personal note, I don't really understand what you're saying. Congestion goes up because there are less vehicles on the road? It sounds like you're trying to play contrarian and arguing for unintended effects and trying to assert something about short trips, but I'm at a loss for how your hypothesis is supposed to work.
