You can say either حادث طريق or حادث سير for singular and حوادث طرق or حوادث سير for plural and in your sentence I believe it should be حادث طريق

Thank you very much. It seems I was taught something different, but as you are the native speaker among us, I gladly changed.

I was curious (as I've always known the expression حادث طرق
so I Googled it (with في). See the results:
https://www.google.com/#sclient...iw=747&bih=498

And now I Googled what you suggested, i.e. في حادث طريق and received only about a quarter (!) of the previous results:
https://www.google.com/#sclient...iw=747&bih=498

my personal opinion on this is that there's nothing really wrong with حادث طرق itself (although it does sound kind of illogical)....but reading something like:
لقي ثلاثة مصرعهم صباح اليوم في حادث طرق...
or
قضيتنا لن تصبح مجرد حادث طرق عابر في صفحات التاريخ
in a book or newspaper sounds fine to me...(and yes you can find it used in newspapers and books in google results)...but in this sentence it doesn't sound right to me, maybe because it specifies that a "person" (singular) was involved that it becomes glaringly weird to use طرق (plural), or maybe because حادث طرق tends to be used in contexts that are inherently neutral in terms of emotion and written in a formal, distant tone in contrast to this sentence that is a more personal, emotional context.

Thank you very much, saeb, for your input.