Booster seats

I was getting behind in my safety knowledge (who, ME?!) and decided to look up booster seat info. Here is what I found:
http://www.carseat.org/Boosters/627_choose_boost.pdf
It says:

Recent research suggests that boosters with a back
are more protective than those without a back, particularly in side impacts.
...but goes on to say:
Backless boosters are lightweight and inexpensive, so they are a reasonable choice for carpooling, play dates, and public
distribution programs. A backless booster should not be used unless the child is able to sit upright throughout the trip and
the child’s head is supported by the vehicle seatback.

I remember the seatbelt rubbing my neck when I was a kid, so I would tuck the shoulder strap behind; I also remember removing the shoulder strap so that I could lay down to sleep in the back seat (with the lap belt around me). I guess Tessa's booster seat "wings" will keep her upright, because now I know better.

Tessa is 47 inches and 44 pounds, so I thought maybe it was time to let her go "backless" but reading this convinces me otherwise. I will waiver to allow for backless on playdates (especially with three in a row, it's hard to fit three booster seats in the back of a car) but we will keep the back on in our car for a long time.

Boosters are required to age 8 or 4'9" - see the Washington State law here:
http://depts.washington.edu/booster/anton_skeen_bill.html

I'm only posting this info because I had to seek it for myself, so thought that someone else might be looking for that info and benefit from it as well.

Edited to add:
Booster seats protect against serious injury 3 ½ times better than seat belts. Booster seats protect against head injury 4 times better than seat belts.
http://depts.washington.edu/booster/faq.html#why