We're lucky that we don't have to drive very often but when we do, we love taking our bikes with us. It's great to be able to extend your biking range or to get your bike to a new location that might be too far to safely/reasonably bike to. It can be tricky to get a cargo bike on the back of a regular-sized car but it's not impossible. Here's what we've done.
My parents have a Prius that we sometimes borrow and we have a Saris 3-bike trunk rack, nothing fancy. We went with the 3-bike rack because it's nice to be able to bring as many bikes with us as possible, and I realized that if a rack is sturdy enough to carry three bikes, it's probably okay for one giant bike. The first time we tried to get a cargo bike on that rack, it was actually the Mundo, an 80ish pound cargo bike. I stripped off everything that could be removed to lighten the weight just slightly, then managed to hoist it up and strap it down with every tie-down I had.
Our next cargo bike-on-car experiences have involved the Haul-a-Day. This bike only weighs about 40 pounds and is shorter but has some unique tubing to deal with. I had originally bought a crossbar adapter for the bike so it could fit across like a "normal" bike but the adapter didn't fit and I didn't want to wait.
For my next attempt, I had had a little more time to decide how to set up the bike and I had to make sure it was secure as we were driving up to Eugene for the next 10 hours. We had a new rack (same one) and about 4 different types of tie downs.
This time, I placed the bike upside down again but with more of the rear weight closer to the rack. This meant that the part of the frame that was on the rack was right in front of the seatpost and the front of the deck. The bike was more balanced in weight and the additional tie downs held everything securely. We drove the whole way at (slow) freeway speeds and the bike didn't move once.
1. Remove as much as you can, especially anything that could fall off if jiggled a lot and anything that catches the wind
2. Find the bike's true center of balance and try to hang it on the rack from there.
3. Use as many tie downs as possible. With the 3-bike rack, I can usually get the tubes of the rack to fit into the slots where two different bikes would go. The snap downs that would usually attach to a seat tube can hook to the seatpost instead. Get creative and then use extra bungee cords and cam straps as extra securing, to keep the front wheel/handlebars from bouncing into the car, and for peace of mind.
4. Unlike with the Mundo, I can actually do this all on my own but it is much easier to have a second set of hands to help.