After working for eleven years as a police officer, I often think I've seen it all, and then I'm hit with a new whammy. There have been assaults on real estate agents all over the country, and one that seems to be making its way to the top is the assault, car-jacking, or robbery of a real estate agent by a passenger in their vehicle.
Here's why this is a whammy... why was the person in the agent's car to begin with? 
Ok, first of all, I know some of you are raising your defenses, about to tell me all about why you ride prospects around with you. You want to listen to their conversations, watch for the "buying signs", etc. Well, to be frank, I take a hard line approach with this.
My rule of thumb is this... prospects NEVER ride with me. EVER! Now, I'm not as buff as I once was (ha!), but I'm still a six foot tall guy who can "take care of himself". I won't break my rule. Why? Well, consider this...
When you allow someone to ride with you, you are giving up a modicum of control to that other person. If they have a weapon and choose to use it on you, or threaten you with it, you are much more at their mercy than if you were allowing them to follow you.
I have done pretty well in real estate over the years, and all without riding around with buyers. Oh, I still watch for buying signs, but I do that when I'm walking around with them at the properties.
Make a decision. Would you take a sex offender for a ride with you? Would you take a robber for a ride? How about a murderer?
How do you know which category they fit in until it's too late? Think on it.
By the way, can I have a lift? Be careful out there!
Bill, You bring up some great points. I can't say that I never ride clients in my car because I have a lot of out of town people with no transportation but I don't take them alone. If they are local buyers, I have them bring their own car.