Although this activity is a little overwhelming at first, it is one of those lessons that gets better every year that you try it. The best advice that I have to offer is to meet with your accident reconstructionist before you invite him/her into your classroom. Take them out for coffee and discuss your vision for the day and how they might be able to contribute ideas, advice, and mathematics to make the experience as rigorous and relevent as possible for the students. It is important to remember that not all people are comfortible taking center stage of a classroom, and meeting with your guest first is extremely important to determine a comfortible role for everyone involved.
Chances are that your experience and activity for the kids will not look exactly like mine. However, I can guarantee you that if you put in the extra work and make the leap it will be great for the students to hear mathematics that is utilized in the real world.
If there is time at the end of the presentation, I like to open the remainder of the class up to questions for the officer. Students will often ask follow up questions about the career of an accident reconstructionist, specific accidents the officer may have talked about in his/her presentation, or other topics that came up in the day. It is important to have the discussion about appropriate types of questions with your students prior to the guest coming in (I do this the day before). Once you have the conversation with your kids, 99% of them are mature enough to keep their questions professional. In the instance that a student ask a question that I do not want the officer to feel pressured to answer, I call on another student for a different question - acting like I did not hear the previous student. However, this has never been an issue in my class... the students ask great questions and I'm sure the same will be true for you!