The blue
reflectors in the
midst of the road are
trapezoidal prisms; that is, an
isosceles trapezoid projected along the
axis normal to the trapezoid. The reflectors are
not placed in the middle of the road; rather, they are placed
offset to one side of the road.
Located on that side of the road is a fire hydrant.
In very snowy areas, the blue reflector may be supplemented by a 3 to 4 foot tall stake adjacent to the hydrant, topped with a circular red reflector. The stake identifies the location of the hydrant for fire departments (who shovel our hydrants after every storm), and for snow plows, to warn them not to plow through the hydrant.