Duel : The number plates on the truck

Were the number plates from all of his victims. Nice little touch there IMO, but I'm surprised that I only just noticed this after watching this movie quiet a few times over the years

No, multiple plates was due to separate registrations for paying varied road tax in those states.
_________________________________________________
Did your parents have any children that lived?

Is it legal to have more than one number plate on your vehicle?

Well yes it was, for commercial trucking it was required. I remember seeing tractor trailers even up to the 80's with multiple license plates on the bumpers.
________________________________________________
Did your parents have any children that lived?

So truckers don't need separate registrations anymore? Sorry for asking, I'm just curious and I'm not from the USA so I'm not sure how you guys do things

The plates were actually fuel tax stamps and proof of insurance(not registrations). Most states had reciprocity, ones that didn't required a separate tag for it. Back then it was all done on paper, now computers run it all.
_________________________________________________
Did your parents have any children that lived?

Ok, got yah

Spielberg said on the documentary the plates all belonged to his previous victims.

Well regardless of Spielberg's perception, there is a practical and logical reason for a commercial tractor trailer to have multiple license plates.
___________________________________________________
Did your parents have any children that lived?

In the commentary to the film Spielberg refers explicitly to the plates as belonging to the cars that the truck had previously "killed", like a shooter from the west counting his victims by the marks on his Colt.

Well there you go, I was right

At first I thought it was souvenirs from his victims but then I learned of the license plates for each state it is registered in. It makes more sense that it is the registration answer otherwise wouldn't the guy get questioned about why he has so many license plates by police etc? And it would draw suspicion if he has license plates of missing or stalked victims. It definitely would be more sinister if it was that reason but I don't think so after thinking about it over the years.

To be fair Spielberg says that he "always imagined that they (the license plates) were like the notches in his '45".
'Imagined' being the key word here, meaning the license plates could be genuine tax/fuel-whatever, but in the imagination of the creator of the movie they are the colt-notches.

Serial killers like to keep trophies.

I remember seeing these trucks with multiple plates back in the 60s.
Don't remember the reasoning, but it wasn't Spielberg's version!
You might even find drivers with multiple licenses.
Legal?
Maybe not!
Until you got caught!