While your girlfriend might use it for her baby and your pediatrician might even have said it's OK, the Baby Bjorn (or any other type of soft infant carrier – Snugli, etc) will not protect a baby in a crash.

Researchers Kathleen Weber and John Melvin of the Highway Safety Research Institute at the University of Michigan Medical School tested this scenario using a 30 mph, front, dynamic crash test of the type required by the current federal safety standard for child car seats. In the crash test, an adult held the baby in a soft, cloth front carrier like the Baby Bjorn and used a lap/shoulder belt. The researchers found that this infant was at a very high risk. The tested carrier shredded completely, ejecting the infant dummy into the dashboard. If the carrier had not shredded, they found that the infant would likely still not have survived. As the adult's head traveled forward in the whiplash motion, the adult's chin would have slammed down into the infant's head right where the soft spot is.

If you find yourself in a taxi with just your infant and a Baby Bjorn (hopefully you never will), there is no way to protect your baby. However, you can still protect yourself by wearing your seatbelt. Putting the seatbelt over you and the baby will only make matters worse. It will not help the baby and will endanger you in the process.