Now Pay Attention !

The dynamic changes to the vehicle fleet will likely continue to pose issues concerning visibility and signing requirements, as is evident with the proliferation of SUVs and minivans versus signing practices to accommodate sedan passenger vehicles. To accommodate the full range of driver eye heights, i.e., the range of vehicle types in the vehicle fleet, including tractor-trailers, more emphasis should be placed on the resulting observation angle.
or
Put the bloody sign where you can see it....