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It would have been funnier, if they had actually ADDED a piece to the speed limit sign, like a zero. A speed limit of "550" would surely be funnier.
In cannonball 2 they added a sign with a speed limit of 155.

Then again, no human has to obey speed limits - only 'persons' do. Persons are artificial, and exist only on paper. It's unfortunate that so many humans have performed 'joinders' between their living, human self, and this paper-'person' that can also be called 'strawman'.
You sound like every idiot screaming that they don't have to pay taxes as they sit in prison for not paying taxes.
HELP AGGRAVATE THE STATUS QUO, VOTE AGAINST EVERY INCUMBENT YOU SEE ON A BALLOT.

By the way, that's some weak paint.
It would have been funnier, if they had actually ADDED a piece to the speed limit sign, like a zero.
A speed limit of "550" would surely be funnier than a weak, almost non-visible, RED X on top of the actual limit (plus a mysterious, added, unnecessary squibble).
I guess women just can't resist adding useless crap to things whenever they can (pointing to the hoarders, porcelain cat owners and such)?
It's a bit unfair to expect the viewer to realize this, though, and not try to decipher the extra marking. How is every viewer supposed to be familiar with the habits of schoolgirls? I sure don't know any schoolgirls.
And those women certainly weren't schoolgirls, they were grown women, so when is it proper to expect them to behave that way? Look, defy the system or try to look cute, but don't try to combine them both and expect the viewer to instantly get it and have no problem with it.
I don't think she should have added the squibble, if she only meant to paint over the "550" with the weak, red paint. From far away, their squibbly x wouldn't even be very visible.
A better way to do it would be to have different-numbered sticker of the same size slapped over the old one. Would look more professional, and be potentially much funnier, and still defy the system.
Then again, no human has to obey speed limits - only 'persons' do. Persons are artificial, and exist only on paper. It's unfortunate that so many humans have performed 'joinders' between their living, human self, and this paper-'person' that can also be called 'strawman'.
http://yourstrawman.com/
Meet Your Strawman:
https://www.youtube.com/watch?v=ME7K6P7hlko
So the whole exercize is pretty much futile. The cops would have to PROVE that they need to obey some act or statute. Instead of performing a joinder, identifying yourself as a 'person' and THEN defying the very acts and statutes you have consented to being governed by, why not simply revoke consent and claim your rights? The cops could forcifully take you of course, but then you could tell them you are coming with them only under protest and duress, and that would actually then create trouble for them later, when you bill them (after all, you had no obligation, and they still caused you duress, and destroyed your valuable time, so they are liable to be billed).
Oh well, a fun movie nevertheless. This scene just always bother me, because it could have been done so much better in so many ways. Why not AT LEAST slip a "one" before the "55"? So it becomes "155"?
(But NOT with a weak, red paint!)

The same beep significance as schoolgirls who dot their "i"s with little hearts. SMFH Why don't you track down the actress, Tara Buckman, and ask her?
"Why must you needlessly complicate EVERYTHING?" Gordon Shumway

"..she's simply crossing out the number 55, nothing more."
If it was nothing more, she wouldn't have made the symbol at the end.
What the OP wanted to know, as far as I comprehend it, is whether the squiggle at the end has a signifigance, or not.
If not, why would she do it that way, instead of a straight line?
If yes, then what is the signifigance?

Yes that is about it all right!

You can call it "stylized" or perhaps even "girly," but she's simply crossing out the number 55, nothing more.

I'll add that the theatre I saw this in is no more, and is now an artificial rock climbing deal.

No one obeyed it unless you were over 80 or defected from Russia or Communist China.
On opening day when I saw this movie people cheered and applauded when the girl in the Labmo got out and spray painted the speed limit sign.

No they would not obey it at all.

I don't know about the "squiggle", but opposing the "double nickle rule" was a very popular form of civil disbedience during those years. Between 1974-86, 55 mph was the highest speed limit anywhere in the US. And it was a riduculous rule, because not many people would obey it.
http://en.wikipedia.org/wiki/National_Maximum_Speed_Law