I respectfully disagree with the commenter who said riding in a trailer is no different than riding in a car. There are far more vibrations on a bike/in a trailer than in driving in a car. Most cars have high-tech hydraulic shocks that absorb the bumps on the road. Bikes and bike trailers may have shocks, but to compare those to shocks on a car is ridiculous. Also, the larger air filled tires on cars do a lot to absorb impacts, and there is really no comparison to bike tires. Whether or not the infant seat is secure in the trailer is really a side argument.
My point, which you didn't address at all, is the use of a helmet. A lot of states require helmets for kids on bikes, and riding in bike trailers. Just because a child is a passenger in a trailer does not mean they will be protected if hit by a car (or flip). Those trailers are flimsy. Yes, they have the "roll bar", but pit an aluminum and cloth trailer against even a slow moving car and I think we can all agree which vehicle will prevail. It is my opinion that putting a child in a trailer (or on a bike) without a helmet is being an irresponsible parent, regardless of your state law. See www.ibike.org/education/infant.htm and www.helmets.org/mandator.htm.
Bike trailers attach to the back of your bike and can be zipped to completely cover baby with a breathable mesh. You can simply strap baby’s infant car seat right in the trailer. When children are a bit older, they can sit in the trailer without a car seat. All baby bike trailers come with five point harnesses for buckling babies or toddlers inside, and most have a full roll cage. Most will seat one or two children.
Some families have put their child in their car seat into the trailer. This practice has not been officially safety tested, but if the child is securely strapped in to the car seat and the car seat properly strapped in to the trailer, it may be the best option for transporting a young infant.