I avoid mass transit when I can, but it is sometimes easier / cheaper for me to use it... W/ the HP tag on my van, I can park free w/ no time limit in most street parking... The garage at the hospital I get most of my care at is $11-20... I can park near a transit stop and use my HP discount "Charlie Card" to get the subway to the hospital, round trip under $5, and only 30-40 minutes longer... At least in decent weather seems worth it. On the subway there is no securement, and you sit whatever way works (some cars have WC parking areas, some don't, and you might or might not be able to get to them if it's crowded) I find my chair lurches back and forth a bit on the drive slop, but doesn't move, however the speed changes are pretty slow.
On the buses (which I try REALLY hard to avoid because of the extra hassle of getting strapped down) we have the 4 straps tiedown w/ driver assist, and that keeps the chair from moving. However when I've carried my chair empty in my van it will slide all over the place if not tied down, same thing on the few times I've tried riding in a vehicle w/o being tied down, or when carrying someone else... It seems to me that a road vehicle does fast enough starts and stops that some sort of securing is essential.
I suspect that this Quantum system is going to be adequate for keeping you held in place during normal riding conditions... In case of an accident you might be

but odds of that are fairly low... In addition busses are big enough that most car hits aren't really going to bounce you around that much... I remember a guy centerpunching my big yellow schoolbus when I was in High School, (he didn't see it

) We barely felt the hit, his car had major damage, the bus had sheet metal and a broken brake line... Another time in front of the place I was living, an MBTA (Mass. Bay Transit Authority, our city bus system) bus hit a car that tried to make an oncoming left in front of it... Car looked like a total, bus had a rip in the rubber strip on the bumper...
So I'd be pretty much OK with this setup as a reasonable solution to the challenge of having wheelers use a bus w/o slowing the bus travel down to much....
ex-Gooserider