I think the way you've done it in your example is fine (except you shouldn't number your scenes unless this is a shooting script)
The characters do remain in the same INT location (the car) but you want to show their slow progress, by showing them stuck in traffic in the tunnel, the different streets, etc. -- you're showing different locations (ext) and then them in the car (int) -- so each time *is* a different scene.
I do think it's easier to read if you separate the EXT/INT action, but then that takes up a bit more room -- b/c you have to have something, even just a few words, in the action description before the character speaks.
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EXT. HOLLAND TUNNEL - LATER
Traffic is at a standstill.
INT. CAR
Alex keeps checking his watch.
ALEX
I don't think we're going to
make the show.
MID-MANHATTAN STREETS - MUCH LATER
Alex drives as fast as he can given the traffic.
JENNIFER
Maybe we should skip dinner.
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You can just indicate LATER when the scene is taking place in the same location --
e.g. an office:
INT. OFFICE - NIGHT
Alex on the phone as he shuffles through paperwork
ALEX
(into phone)
I'll be home soon, I promise.
LATER
Alex munches on a take-out burger while he goes through the
stack of files.
LATER
Alex snores gently, his head on the desk.
The phone RINGS and he jerks awake.
ALEX
(into phone)
Uh yeah, s'me...what? Ten-thirty!
He looks at his watch. (etc. etc. )
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