"While I have found that this type of behavior is relatively common among drivers..."

Cyclists may be more aware of how common this behaviour may be - personally I would not volunteer this piece of information, and would infact atempt to imply the opposite, thus justifying outrage e.g. "in my many years of cycling I have never been subject to such aggresive and truly dangerous behaviour".

"It also begs to question..."

Shouldn't that be "It also begs the question"? Google returns 1,700 hits for your usage and 1.7 million for the latter.

"As I previously mentioned, I experience this type of behavior on a regular and frequent basis..." see earlier suggestion.

"I realize that there is likely no way to be able to identify..." I agree with other posts that this may not be a valid assumption. I would encourage that no effort be spared in finding and admonishing the culprits.

Your letter should include a comprehensice CC list including publication on numerous web sites (actually list them) and submission to all local news papers etc.

In general an excellent letter, sorry to hear that you also have been on the receiving end of this kind of reckless and totally infantile abuse - I hope it costs him/her/them their jobs. You might also want to mention the financial liabilty that these miscreants were potentially exposing the state to.