It's simple physics - the attached carriers are not safe.
i just found this thread, otherwise i would have commented much earlier.
this is a totally unqualified statement, and i will comment on this further. i have done much research into this matter and it is not as simple as you say. there are safety issues regarding all types of carriers, be they rear-mounts, front-mounts, or trailers. i would suggest you read up on these issues before making such a broad and inaccurate comment.
i'll reply more in-depth later today, as i am off to work now.
EDIT: okay, i'm back, and i read the whole thread again.
skater-- in all of your replies, you never once provide a valid argument why rear-mount carriers are unsafe, other than "it's simple physics". the fact is, there is a big misconception about their safety and about the physics behind them. most people think that they skew the center of gravity of the bicycle veritcally, making it more top-heavy. this is not true. a properly installed carrier does not *raise* the center of gravity significantly, because the child's vertical center of gravity is around where the rider's vertical center of gravity is. so the bike is no more "top heavy" with the carrier. what the carrier *does* is shift the center of gravity more to the rear, which does cause a slight destabilization. on the other hand, there is also a benefit to having the center of gravity shifted more to the rear, in that it actually stabilizes the bike during emergency braking.
while trailers do not alter the center of gravity of the bicycle, they have their own dangers that rear-mount carriers do not have. for one, they create a much larger footprint on the road. if you are sharing the road with traffic, this creates an immediate increase in danger, since you are a much larger target. this is mentioned again and again by drivers in urban areas. also, bike trailers may appear more stable, yet they actually have a high rollover risk when hitting bumps. for example, the turn radius is much larger with a trailer. if a cyclist is making a right hand turn and comes close to the curb, the right trailer wheel may hit the curb and cause the trailer to flip. this is a proven characteristic of trailers. rear-mount carriers do not affect the bicycle footprint and the bicycle is as maneuverable with or without the carrier. this is important in urban environments.
so it seems that both methods of transporting kids on bikes present their own risks. rear-mount carriers are usually preferred in urban environments where there is a lot of street riding and the roads are shared with motorists. trailers are preferred in rural areas where there is little to no traffic, or on controlled access bike paths.
i would highly encourage ANYONE who is interested in transporting kids on bikes to thoroughly learn the benefits and risks of each method, and to also make sure you buy a CPSC approved toddler helmet. the best source of information on the subject that i've found is
http://helmets.org/. you can learn all about proper helmet use and the debate between rear-mount, front-mount, and trailer carrier methods.
this link within helmets.org provides links to actual medical journal articles as well as other links outlining the safety risks associated with each carrier method.
to make the blanket statement that they "are not safe" is innaccurate and uninformed, analogous to claiming that RFing beyond 1 year is unsafe because the legs can get broken. it appears that way on casual observation, but there is no data to back it up.
that said, this is the carrier we just upgraded to. we had a really cheap BELL seat which DD1 outgrew by height. this new one is german-made and is rated to 50 lbs. all the others sold in the US are only rated to 40 lbs.
http://www.bicycletrailers.com/Kettler-Teddy-Bike-Seat.pro
to answer the OP's question:
here are some things to look for:
- some type of suspension to minimize the shocks from bumps. AFAIK, only the topeak and the kettler have real suspensions. (incidentally, in europe, ALL carriers must have their own suspension, and ALL must be rated to 22kg, or 48 lbs). the standards are much more relaxed in the US.
- weight limit: all seats sold in the US except the kettler go to 40 lbs (the US-mandated limit), the kettler goes to 50.
- foot straps: these keep the child's feet strapped into the foot holsters. this prevents them form kicking you, but more importantly, in a fall, the feet and legs are protected by the seat shell. (these are required in europe, but not in the US).
- adjustable headwings. most seats do not have these and leave the head exposed during a fall. BELL has a "cocoon" model which has a nice tall shell that surrounds the head, and the kettler has adjustable headwings much like a carseat.
another thing that is
very important to note is that there are a LOT of incompatibilities with rear carriers and different bicycles. not all carriers will fit all bikes, and if you buy one off the internet without first trying it on your bike, you may be disappointed. you really need to go to a bike shop and try out some different carriers to see what fits your bicycle.
also, to the OP, while it is technically legal to ride your 13 mo in a carrier in most states, i wouldn't recommend it. not from an accident risk, but from the standpoint of risk of brain injury from bumps-- and this applies to trailers as well as rear-mounts. if the child's neck muscles aren't strong enough to support the head for extended periods of time, injury comparable to shaken-baby syndrome can happen. it's unlikely after 1yo, but helmets.org recommends against starting your kids in a carrier at 1yo. we started DD1 at 2.5 years, and we will start DD2 at 2yo this june.