My son is 4'4" tall, and weighs 60lbs.
As parents, we seem to be the last of our peers to still sit their 8-year-old in a booster seat for any duration of car ride.
General booster seat guidelines for Canada and the United States recommend a child weigh at least 40lbs, and reach at least 4'9" before they should be allowed to travel in a family vehicle without a booster.
Without his booster seat, the shoulder belt rests on my son's neck - an added danger should we ever be involved in a front-end collision or stop suddenly. Laws and manufacturer recommendations cite this risk specifically when they outline height requirements.
Proper Use of a Booster Seat - National Highway Traffic Safety Administration
At 4'4", my son is among the tallest of his friends. However, his peers and their parents have begun to comment on the booster seat:
"You're still in a baby seat!?" the kids will ask.
"Oh, he's still in a booster?" their parents will echo "We usually don't bother, unless it's a long trip."
I have begun to allow him, when he's in his friends' cars - or even when his friends ride with us - to leave the booster seat behind. By doing this, I'm allowing my empathy for his embarrassment to trump my concern for his safety.
I imagine other parents made the decision to throw out the seat under similar conditions.
Different model cars do fit children differently, thus not always necessitating a booster. We have a compact vehicle, in which my son rides without his seat because the seat back is relatively shorter and the belt fits him properly; we also have a van, in which the belt would act as a choker were it not for the booster.
However, in our suburban neighborhood, most parents drive full-size vehicles or vans. I know, because I'm loaded my booster-less son into them.
It's possible parents are uninformed. However I remember conversations with my peers as we all became first time parents: nutrition, sleep, and car seats were all heavily researched topics.
It is more likely boosters have been left behind due to external peers pressure from children and parents.
I guiltily protect my son from embarrassment on a case-by-case basis by leaving his booster in the garage when he rides with friends.
As with any safety issue, this is working out just fine, until the thing that 'never happens to my kid' happens. If it does happen, that booster seat sitting silently on the garage floor will become a sad metaphor for a parent who wouldn't speak up.