With these things it may be a case of deciding what outcome you are looking for.
I know quite a few people who are quite reasonable and bright, but who would really not respond well to a polite email criticising their driving.
I also know (as we all do) quite a few people whose idea of a polite email is not everyone's idea of a polite email.
Email is a comms system to which mankind has adapted slowly. I think it is often the WORST option for a first contact with someone, particularly if there might be some whiff of implied criticism in its content.
What is the outcome you'd like to achieve?
If it's to risk p*ssing the recipient off, go with the polite email.
If it's to give them something with your sig block on it that they can send on to a gazillion other colleagues, go with the polite email.
Even Shakespeare or Waugh (not the cricketer) would have difficulty wording an email that was proof against either of the above scenarios.
I don't see the fact that this person works at your office as being significant here, nor the fact that they have some seniority.
If they are doing something so terribly naughty that it's reportable, I'd report them. If they are serial offenders, others will report them too.
If they are mildly annoying or not yet used to the width of their vehicle, they may just be in that minority of road users sent here to annoy other road users.
Good question; but however tempted you are, I'd stay well away from email for a case like this.