In order to increase fuel efficiency, one of the things car companies do is replace heavier steel parts of the car with lighter metals and alloys. Lighter weight makes it easier to propel a car using less energy, and hence less fuel, but it also makes the car more susceptible to damage from high speed impact.
I believe that safety concerns are involved when it comes to increasing vehicle fuel economy standards. As anyone who studied physics knows, larger cars are safer cars. Generally, larger cars are also less fuel efficient than smaller cars. Attaining fuel efficiency often means making the car smaller, which makes it less safe in a collision.