In the CPS world simple rules are made that result in the best protection for the majority. However the real life variables are much more complicated and often we just don't know for sure what is best. We can look at the big picture and make an educated guess, but that's all.
The rationale for the under 13 rule is protection from air bags. Real life data indicate that air bags are detrimental to unrestrained children under 13. For unrestrained children 13 and older they provide some benefit. (For restrained children there is no difference for ages 6-8 and benefit for those 9 and up.) Note that this is compared with the front seat with no air bag, NOT with the rear seat. Basically the rule is designed ONLY to provide protection from air bags.
If you are comparing front seat with air bag to rear seat safety IN GENERAL, everyone under 50 is safer in the rear seat. People over 50 should sit in the front with an air bag.
See how the under 13 rule works? It provides added safety to restrained children under 6 and unrestrained children under 13, while having no detrimental effect for restrained 9-12 year olds who are just as safe in the back as they are in the front with an air bag anyway. But the simple rule is much easier to remember and follow.
Now for the petite adults. Here's a link to an NHTSA study comparing rear seat protection to front seat protection.
http://www-nrd.nhtsa.dot.gov/pdf/nrd-01/esv/esv19/05-0212-O.pdf
Small adults are likely safer in the front than in the back. On pages six and seven, figures eleven and thirteen there are charts comparing front passenger and rear passenger crash test results in five newer vehicles with a restrained 5th percentile female dummy. Note that the front seat has much better numbers. The average adult also had higher numbers in the back, but these were within acceptable limits. So from now on, if one parent needs to sit in the back, make your husband be the one.
Quote from page 8: "This suggests that the advanced restraint systems of the front seats in these newer vehicle models make the front seat position more effective than the rear seating position for adult occupants in reducing serious to fatal injuries." (Kuppa, 2005)
Page 8 also explains that 64 percent of rear seat occupants are under 12, but they account for only 32 percent of the serious injuries and 26 percent of the fatalities. So the fact that children usually sit in the back is one of the reasons that rear seats appear safer in general, but this may or may not be the case for adults.
I also like the interesting wave you can see in the real world fatality data on page 2 comparing restrained rear seat passengers with restrained front passengers with an air bag. You can see the rear seat provides the most additional protection for small children and young adults, slight or no benefit for nine to twelve year olds, and much less protection for those over fifty. It seems the rear seat belts are not as kind to more fragile bodies as the front ones are.
The ironic thing is that many parents think "My child is big for their age so they can ride in front," while what little data we have indicates that if they're big for their age they're definitely better off in the back. It would be petite teenagers that might be better off in front--the 85 pound 13 year old could possibly benefit from sitting in the front, while the 150 pound 11 year old (or 13 or 15 year old) is better off in the back. On the other hand, if the air bag turns off when the 85 pounder is sitting there, the back seat is the way to go.
This article is encouraging development of dynamic crash tests for rear seat belts that will hopefully result in more cars having pretensioners and load limiters in the back making it safer for everyone. I would think it would be a long time in coming though.
Sorry I'm being as clear as mud. But that's why we learn the simplified rules in the first place.
Julie D.
Another link:
A Transport Canada study on booster effectiveness also notes better protection for the small adult dummy in the front as compared to the back. This one focuses on seat belt geometry. It's also a good read when thinking about extended harnessing of children.
http://www-nrd.nhtsa.dot.gov/pdf/nrd-01/esv/esv19/05-0258-O.pdf