We used to use a 6 point harness in helicopters when I was in the Army.
Jet fighters use the same to withstand the high forces.  I would assume
that the reason they are not used in public transportation is convience.
They are not quick to get into.  Pretty easy to get out of though, just
twist the release and all belts are released.  The use of a multiple belt
system should (in theory) distribute the force more evenly.  I would
highly caution against them though for pregnant women and children.  You
must ensure proper fit of any belt to acheive maximum safety and prevent
injury though.  Good luck.  You will look like and Indy driver in the
world's slowest formula Van!  CU