I have a Ridesafer vest for my 3.5 year old. He's around 33lbs. I would be weary to use it on trips longer than 10-15 minutes. It's technically a booster and doesn't prevent the child from moving around like a car seat, so if you have a fidgety child or if he falls asleep and falls over, it won't be much use.

If you do decide on the vest, order it well before your trip and practice using it. I've only used it a handful of times and the first time took me forever to slide the belt in the guides.