Boosters are better than they used to be at fitting lap and shoulder belts on 4 to 8-year-old kids to restrain them in a crash - so parents don't have to search as hard for a good fit for their child and vehicle. Most belt-positioning boosters, though, don't offer consistently good fit in all vehicles. This is the bottom line in the Insurance Institute for Highway Safety's third round of booster evaluations. Researchers assessed the safety belt fit of 72 boosters, assigning the best ones the top ratings of BEST BET or GOOD BET because they correctly position belts on average booster-age kids in most vehicles. The worst performers are ones the Institute doesn't recommend because they do a poor job of fitting belts. A good booster routes the lap belt across a child's upper thighs and positions the shoulder belt at midshoulder.