Why aren't flashing-light signals and gate arms installed at all public grade crossings?

In Michigan the type of signal installed at a grade crossing is based on guidelines which come from the "MDOT Guidelines for the Utilization and Installation of Traffic Control Devices at Highway-Railroad Grade Crossings" and the "Michigan Manual of Uniform Traffic Control Devices" (MMUTCD). Just like highway traffic signals are not installed at every highway intersection, flashing-light signals are installed based upon current conditions. The decision to install a particular type of signal is based typically on the average daily vehicle traffic (ADT); average daily train traffic; the availability of adequate clear vision triangles; the crossing's accident history; number of tracks; and recent residential or commercial development in the surrounding area.