When we first bought the air navigator, our then 2 year old kept trying to get off the trike - and we walk around on lots of city streets. So, we ordered a seat belt, which was very easy to put on. The seat belt keeps him on the trike, thus making us feel a little safer knowing he cannot get off and bolt into the street.
Please do not turn your back even with the seat belt affixed the the seat and your child. The belt is a nice safety feature, but really is a mere deterrent for your little escape artist :)