Maybe the person was getting increasingly frustrated at the long wait in line, and let out the frustration behind the wheel. (And no, I don’t think that’s a good idea, just a possible explanation.)
Well to be fair, I’m sure you’d get that response on the escalator too.
If you are caught in traffic, you are traffic. Ride a bike.
Not so much. I guess because escalators are self-cleaning.
Just don’t do it on a treadmill while a plane is taking off.
I hadn’t really thought that through, but now that you mention it… I could probably roll up a pair of jeans to a small enough diameter to suck 'em through.
Who am I kidding? It’s just an insurance claim waiting to happen. The 12-year-old boy who lives inside my head would be trying to see what all could get through those things. I’d probably start with grapefruits and work my way around to the cat. 
I *hate *driving. If it were legal, I’d run over little old ladies if it meant I could get out of my goddamn car 5 minutes faster.
Oh hell yeah. Especially those inconsiderate old ladies with their walkers in the intersections. Do they think I can sit there all day while they take their time getting across the damn street?
To seriously answer the OP- in a line at the store you are a vulnerable little human with no protection, on security camera with store managers around.
In your car, you are surrounded by 2 tons of steel with bumpers, 200 horsepower, and door locks. And you are pretty much anonymous since nobody can really see your face. Cars give people an invincible “powerful army tank” mentality.
IOW, what I said upthread when I referred to the Greater Internet Fuckwad Theory.
My father, my BIL, and I are foul mouthed aggressive drivers who absolutely have no patience with speed limits; they are for less competent drivers.
My sister loves to tell the story about she and my mother going on a shopping trip in Fort Myers, Florida at the peak of tourist season. When they returned home, my father asked how the traffic was and my mother said it wasn’t bad and not once did they encounter a stupid bastard or a god damned son of a bitch. I don’t know why she tells that story every time I drive her somewhere.
When an angry person is in line, they’ll keep quiet because if they displayed their anger verbally, everyone else would likely join in to hush them up and tell them to quit being rude. Moreover, the cashier wouldn’t complete the current transaction any more quickly, so displaying the anger wouldn’t help any.
When a driver is angry, they’re anonymous and in more of a one-on-one situation versus the source of their anger. Some intimidation can be used to get the parade maker to yield way
As to why it’s possible not to be angry at 0 mph versus 30 mph, the anger at 30 mph is caused by a perceived violation of protocol. In a 30 mph zone, drivers expect other drivers to travel around 30 mph. In a grocery store, shoppers may not expect other shoppers to travel 30 mph, and may be fine with moments of stasis or nonmobility.
My car is my penis.
I am a dickless wonder without it.