Child car seats reduce injury and death substantially when used correctly. Studies of real-world use have repeatedly found high rates of installation and usage error — figures in the majority in several observational studies.
Most errors are minor. Some meaningfully reduce protection. Knowing which is worth the ten minutes.
The errors that matter most
Harness too loose. The most common significant error. In a collision, slack in the harness allows the child to move before being restrained, increasing forces and the risk of ejection.
The standard check: with the harness fastened, you shouldn't be able to pinch a horizontal fold of webbing at the shoulder. If you can, tighten.
Chest clip position, where applicable. On seats with a chest clip, it should sit at armpit level. Too low allows the straps to slip off the shoulders.
Bulky clothing under the harness. Winter coats compress dramatically in a collision, leaving substantial slack. The recommendation is to remove the coat, secure the harness, and place the coat over the top or use a blanket.
This is one of the more consequential and least known points.
Seat not secured tightly enough. The seat itself shouldn't move more than a small amount at the belt path when pushed firmly. Excessive movement means it isn't properly installed.
Wrong belt path. Seats have different routing for rear-facing and forward-facing use, marked on the seat. Using the wrong one is a serious installation error.
Harness slot height. For rear-facing, straps should come from at or below shoulder level. For forward-facing, at or above. Reversed, the geometry doesn't work as designed.
Moving to the next stage too early. Discussed below.
Rear-facing for longer
The recommendation most changed in recent years and most inconsistently followed.
Rear-facing seats distribute collision forces across the whole back and support the head and neck, which matters because young children have proportionally large heads and immature neck structures.
Guidance in many countries now recommends rear-facing for considerably longer than previous practice, frequently to fifteen months minimum under some regulations and to the maximum limits of the seat where possible.
The common objections — legs looking cramped, wanting to see the child — don't reflect risk. Children are comfortable with bent legs, and leg injuries in rear-facing children are not a documented concern in the way head and neck injuries in forward-facing ones are.
Stages and when to move
The general principle: move to the next stage only when the child exceeds the limits of the current one, not when they reach a birthday.
Each stage provides less protection than the previous. Moving early trades safety for convenience.
Height and weight limits are set by the seat manufacturer and are specific to the model. A child outgrows a seat by height typically when their head is within a specified distance of the top, or when shoulders exceed the top harness slots.
Booster seats should continue until the adult belt fits properly, which is later than most people assume. The test: the child sits fully back, knees bend comfortably at the seat edge, the lap belt sits low across the hips rather than the abdomen, and the shoulder belt crosses the middle of the shoulder and chest.
Regulations and standards
Standards differ between countries, and seats approved in one jurisdiction may not be legal in another.
Newer standards in some regions classify seats by height rather than weight and include side impact testing requirements.
Worth checking which standards apply where you are, and whether a seat bought elsewhere is legal and appropriate.
Second-hand seats
Generally advised against, for specific reasons rather than general caution.
A seat involved in a collision may be compromised in ways not visible externally, and you cannot verify history.
Plastics degrade with age and UV exposure, and seats have expiry dates for this reason.
Manuals and parts may be missing, and recall history may be unknown.
An exception some people make is a seat from someone they know well, where the history is certain and it's within its expiry date.
Getting it checked
The single most useful action. Many retailers, fire services, and road safety organisations offer free car seat checking.
Given how common installation errors are, and how difficult they are to detect yourself, having someone competent look at it once is worth considerably more than reading about it.
It's also worth rechecking after moving the seat between vehicles, which is when errors are most likely to be introduced.
General information only. Follow the manufacturer's instructions for your specific seat and vehicle, and current guidance from your national road safety authority.
Airbags and front seats
One safety point serious enough to state separately. A rear-facing seat must never be placed in a front seat with an active airbag.
An airbag deploying against a rear-facing seat can cause fatal injury. Where a front seat must be used, the airbag has to be deactivated, which on many vehicles requires a switch or a dealer procedure rather than simply being assumed.
More generally, the rear seats are safer for children than the front, and the middle rear position is frequently the safest of all where a seat can be installed correctly there.
The vehicle handbook covers airbag deactivation and permitted seat positions, and it is worth reading the relevant pages once rather than assuming.