Traffic laws are ones of those laws that are difficult to enforce.
Can I say "ones of those" when the subject is plural?
"ones of those" is non-sensical and clumsy. The only time "one" can take a plural ending is when it is a pronoun used to avoid repeating the same noun twice, in which case it must have a plural referent:
I like the red gloves, but I don't like the green ones.
(= green gloves)
In your sentence, there is no such plural referent, so this is not a pronominal use of "one," in which case it can only be a numerical use (=one of several), and "one" as a number, by virtue of it's logical and inherent singular nature, can never be pluralized, unless you're referring to actual written numerals, e.g. be sure to underline all the ones, or, in the US, $1 bills, e.g. I have a five-dollar bill and four ones.
Furthermore, in your sentence, the subject is the plural noun traffic laws, and a plural noun cannot be effectively singularized to comply with "one of those." You would have to use a different approach:
Traffic laws are an example of laws that are difficult to enforce.
-or more creatively-
Traffic laws are notoriously difficult to enforce.
Traffic laws are among the most difficult to enforce.
A better way to communicate your meaning would be to say "Traffic Laws are some of those laws that are difficult to enforce.