[aquaphile]

You get both: isofix/latch as well as lap-belt. Each car seat is sold with an accompanying Latch (aka Isofix) base so you can roll with whatever you prefer. However, lap belts are ubiquitous and work really well.

Reasons to use a base:

1) Convenience. It is nice and fast to click-in, click out with a car seat. Super fast and easy.

2) Protect the seat cushions of the car.

3) More constraints on pitch rotation. Which can be good or bad depending on how the seat is designed and rotation is used.

Reasons to use a lap belt only (no base):

1) It is intuitive. Everyone -- including grandma, grandpa, and the babysitter -- knows how to use a lap belt (as opposed to a latch/isofix base).

2) It is ubiquitous. Every automobile and plane seat has one. So if you're hopping into an Uber, no problem.

3) Lab belts are designed to stretch which is actually really good in a collision. The stretching lowers peak acceleration, and therefore lowers the likelihood of injury.

4) Total system weighs less, which translates into less force in a collision (F=ma).

[bdavbdav]

Many thanks for the thoughtful reply! Coming from Europe here where it’s been on every car for about 20 years so has become very much the norm.

Belt (1) troubles me slightly in that it’s easy but not necessarily intuitive enough for grandma to get it right every time (and indeed many don’t). The base has the great benefit of being definitively installed correctly (all goes green / stops beeping).

The reported numbers on belt errors are pretty terrible: https://www.besafe.com/child-car-seat-misuse-study/

[aquaphile]

A good dialogue is always fun. Thanks for bringing some science to the thread.

You've hit the nail on the proverbial head regarding misuse. Misuse is a problem across installation types: belt-only, and with Latch/Isofix. Some people get so confused they install with both methods.

Lowering misuse is a top design goal. Stated differently: we want to make things so simple that people have to work hard to make a mistake.

You are absolutely correct that belt misuse is a problem (per the cited GDV study). Latch misuse is still a problem too, though.

The studies make clear that misuse is common across installation types, to various degrees. The studies don't do a great job of exploring why the misuse occurred (The 2005 NHTSA study below did ask some good follow up questions). For example, Why did someone not use the belt path correctly? Was it because the slot was too narrow? Was it not visually obvious? Why did someone install both the Latch/Isofix anchors onto the same mount point? Why did someone leave too much slack in the Latch/Isofix anchor or the seat belt? Etcetera. The reasons why people misuse a seat are very valuable to improved public education and improved product design.

NHTSA's 2005 large field study found 39% of CRS (aka baby car seats) were incorrectly installed with Latch. Many of those had multiple errors (e.g. twisted belt plus latch connector turned upside down). Table 11 has the details in https://www.nhtsa.gov/sites/nhtsa.gov/files/latch_report_12-...

10 years later, NHTSA's 2015 study with NCRUSS data found misuse was still persistent: "Overall misuse is considered as having at least one defined misuse present in the car seat or booster seat – the seat may have one or multiple misuses, where one misuse has the same contribution as multiple misuses. The overall misuse is estimated to be 46 percent with a 95 percent confidence interval ranging from 39 percent to 52 percent. By car seat or booster seat type, estimated misuse rates were 61 percent for forward-facing car seats, 49 percent for rearfacing infant car seats, 44 percent for rear-facing convertible car seats, 24 percent for backless booster seats, and 16 percent for highback booster seats." Source: https://crashstats.nhtsa.dot.gov/Api/Public/ViewPublication/...

So for all those parents out there, read your seat's user manual. The user manual will make it clear how to properly and effectively use the seat belt or the Latch/Isofix anchorages.