I'd take this important question to therapy to someone who is adept at helping people with this specific issue.
Driving: While this hasn't been a repeat issue so much as somatic (headaches/severe dizzy to the point of almost fainting) stuff, I find that turning on cold A/C or opening window if winter is a way to turn off dissociation. Even if you are shivering, it will break the dissociative "spell."
Once while driving during a heavy flashback time, I had a positive flashback/switch to a child as I was driving home. While during negative, horror flashbacks, I saw in black and white once, during this positive child time flashback, the trees looked so beautiful and green, the sky so deep blue and wonderful, and the sun on the houses and everything appeared so glorious. It was so beautiful and blissful, like endorphin releasing wonderful, that I couldn't make myself turn away and just focus on the driving. I had to fight hard to stop noticing the beauty and just focus on driving. Later, I was taught about using cold water or cold air, which helps a lot while driving.
Working: I find talking to some trusted coworker or even texting my spouse, will help turn on my "adult-relating" self again and help me not lose time. Again, drinking a cold water or healthy drink will help. Sitting too long is bad for me. Taking 'get up and move' breaks helps me. Again, I can walk to the next office and begin a brief meeting with a co-worker on a project.
Public: When I am triggered to a child state out shopping, it's really hard for me to find a coping skill like the above. I rely on having a routine to find things that work. When triggered, I just send my spouse to do the shopping. Honestly, I'd probably buy everything online often without him. :)
I am still working on this, too. I hope you find some good methods for you. Hugs, Muse