http://www.pscwa.org.au/getdoc/c563e...--July-09.aspx
Based on pelvic restraint, head excursion and abdominal injury potential, results from this study suggest that in frontal impact at least, accessory child safety harness systems provide no better protection than lap/shoulder belt systems. When accessory child safety harness systems are misused the level of protection provided is seriously degraded. Given the high frequencies of child safety harness system misuse seen in field observation studies, and the fact that even when correctly used these system provide no improvement over a lap/shoulder belt, it seems likely that the risks of injury outweigh any perceived benefit of a child safety harness system over a lap/shoulder system.
From Kidssafe
http://www.kidsafevic.com.au/images/...restraints.pdf
Harnesses
Recent research by Bilston has demonstrated that almost 100% of harnesses are fitted incorrectly; therefore, Kidsafe only recommends use of a harness in cases where a lap only belt is the only available alternative.
CR 173: Children in Adult Seat Belts and Child Harnesses; Crash Laboratory Comparisons of Dummy Responses (1997)
The results indicate that the simple addition of a harness system to a lap belt, although reducing excursion of the head and torso, may lead to neck forces that are much higher than those seen in the lap/sash belt configuration. There are therefore grounds for concern on the performance of the lap belt/child harness configuration. The comparatively high readings for head and neck forces and accelerations indicate the need for some attention to design. Unlike the configuration in a forward-facing child seat, the shoulder straps of the child harness when used with a lap belt are anchored directly to the vehicle structure. In a child seat, the harness is attached to the seat and the seat attachments are separate. The lap/harness configuration is therefore much stiffer in its reaction to crash forces, and this may be why the dummy neck loads were higher in these tests. This suggestion is supported by outputs for chest accelerations, which were also highest in the lap/harness configuration.
Road Safety Tips Michael Paine does research on Child Restraints
Except in special circumstances, avoid child harnesses that work in conjunction with a seat belt. Even though these are Standards-approved they usually lack a method to stop the lap portion of the seat belt from riding too high on the child's abdomen. Crotch straps serve this purpose with the harness that is built into a Standards-approved child seat. Some modern designs of booster have an "anti-submarining clip" that is designed to work like a crotch strap. If necessary, these can be used in combination with a separate child harness in centre rear seats that only have a lap belt but make sure the anti-submarining clip holds the lap belt in place. All Australian cars should have a top tether anchorage in this position that can be used for attaching the harness.
http://www.vicroads.vic.gov.au/Home/...rs+(32+kg).htm
The harness must be correctly installed in your vehicle and adjusted properly to fit your child. If it is not, the harness may cause your child injury in a crash.
Harnesses are best used:
* In a seating position where there is a lap belt only
* In the side positions, using the buckle provided, to give extra support for the upper body on long journeys
* When travelling in taxi (harnesses are very portable)
* When transporting extra children and only a lap belt is available.
Photo of Harness where there is a lap belt only: