alt Hacker NewsPortable booster seats are pretty small. I can’t see it working if you have a kid younger than the booster seat min though. Only a few states have strict rules here, Washington and California being a couple, although I think California has a taxi exemption.
Living in San Francisco I've not seen the taxi exemption for California anywhere. New York City taxis are exempt like most of Europe.
My kid is too young for a booster city. It's pretty impractical to take a child seat or booster with you unless you plan to hire a car the other end. When I travel back to see my family in Europe I take the train from the airport. So we bus and Bart to the airport even though it would be much easier with luggage to take a taxi.