I drive a Turbo/stealth flash all the time. I've recently wasted 600 certs reducing it's respawn time; necessary because my righteous use of turbo tended to leave me waiting for 1m30s for another before I'd even left the base.
The last patch or two I think messed up Flash vehicle roadkills. It's quite difficult to have them register if enemies are moving to your right or left. You have to 'lead' them slightly in whatever direction they are travelling. If they are stationary driving through them works.
The best places for Flash roadkills are places where enemies are trapped in a narrow corridor they are funneling out of. For example if a spawn room exit has one of those rails right outside it. Another is the North road into The Crown; though lightnings are best at that because they can shoot you off your flash pretty easy. In a lightning I ran over what seemed to be at least 50 TR who had a sunderer parked at the bottom of Crown on that road a few weeks ago. It was awesome, they were mostly HA and they all just froze in horror as I plowed down the road because they couldn't launch rockets without hitting the guy in front of them. It was one of those moments where they were so closely packed you can't help but kill a ton of them that rarely surfaces in PS2.
I REALLY wish the devs would do one thing for flashes. Have flipping over kick you out of the flash and possibly smite your character with some damage, don't have it take damage upside down, and let you flip it back over. The other day I was making a group of TR look like a bunch of fools with my flash, several tanks and a lot of infantry were shooting at me and I was dodging it, spinning in circles and doing sweet jumps with turbo and stuff. It's actually useful to the team, I swear, because if they are shooting at me for easy points they aren't shooting at the guys coming down the road at them. Anyways though, I was dodging and laughing and only taking a few bullet impacts and small splash damage to my personal shields and flash. Then I landed after a sweet turbo jump, bullets and tank shells going past me, when my right wheel hit a small bump and I flipped over. I slid upside down with my character sliding through the ground, hoping enough turbo would recharge that I could slide my way onto a bump in the terrain and flip myself while airborne, but unfortunately the flash exploded before that.
True, kicking me out and having the flash slide 50 feet away would still have gotten me killed from enemy fire while I ran to it, but it's more for all those countless times I've been stranded on Amerish because my flash flipped over and exploded. In addition the hapless bystanders gawking at me that I've ran over while turboing straight ahead and upside down in that desperate window before my flash explodes might still be alive to this day.