For many years I mistook metaphors and similes, and I'd like to point out the difference here in case it can help you shave some of your learning curve (this is not "splitting hairs", knowing what you're using will help you write better).
The metaphor is implicit, the simile is explicit. An example is better:
"The sky is crying" - Skies cannot actually cry, they are not alive beings and do not have organs to generate tears, etc... This is a metaphor.
"When it rains, it looks like the sky is crying" - This one is a simile. It's easy to detect because you are actually saying X "is like", "is similar to..." Y".
Regarding your text, I like the head=highway of thoughts idea. I think you should perhaps simplify the phrasing; a literary device like the metaphor asks the reader for an effort of imagination, so the delivery around it must be very clear and terse. For example, I concur with what others have said, saying that the highway is "worldwide" blew my fuse as a reader, I was like, "ok, I got that the head is a highway and the cars are thoughts; but then where I put "world"?"
When you write poetry you have to watch always with one eye the idea you want to communicate, and with the other the musicality of sentences. I think you should work more the musical side, and don't be afraid if the "music" forces you to change some of the initial ideas you had.
For example, still with the first line, "Inside my head is an interstate highway", you can try the musicality of combinations like:
Inside my head there is an interstate highway
There's a highway in my head
The interstate highway is in my head, roars in my head, etc...
(A good test is reading the sentences aloud; some poets even record themselves to test the effect).
As for mentioning cars explicitly or not, both options are possible. If I wrote this poem, I would describe different thoughts as vehicles ("some are full of noisy families", "others are slow trucks that never seem to leave the lane"...) Of course this would be my poem, not yours, but I hope some of these ideas inspire a creative spark somewhere...