A sustainable fleet is also a safe fleet.  It is not enough for a fleet to be low carbon, or even low carbon and low emission.  It must also be safe.  And the same applies to the fleet of vehicles that you don't own but are driven on your behalf: the Grey Fleet.  So setting standards for safety is important and the New Car Assessment Programme (NCAP) is a good starts.  Unfortunately it does not cover vans and so you will need to draw up your own safety standards for those.
Of course a safe vehicle needs to be driven safely and as well as ensuring your employees are driving about in vehicle that will offer good protection in the event of a collision it is also important that they know how to avoid an accident by driving defensively and to avoid tiredness by taking appropriate breaks.