GMAT Critical Reasoning (CR) | Re: If only the people using a carpool and only small cars with

If we negate B:
many People use carpool.Now the argument might still hold.As there may be other factor contributing to traffic congestion like large cars which do not use car pool.
If we negate E:
Roads are built to cope with heavy traffic. Now this does not help either.The argument assumes that the only way to reduce congestion is by car pooling or allowing small cars to ply on roads.So whether roads are built to cope with high traffic or not is not much relevant.
I believe that the real assumption