(1)    No person shall park, stop or leave standing any vehicle, whether attended or unattended, upon the roadway of any highway outside a business or residence district when it is practical to park, stop or leave such vehicle standing off the roadway, but even the parking, stopping or standing of a vehicle off the roadway of such highway is unlawful unless the following requirements are met:

      (a)    An unobstructed width of at least 15 feet upon the roadway of such highway must be left opposite such standing vehicle for the free passage of other vehicles. This section shall not apply to a school bus when the school bus is loading or unloading pupils or other authorized passengers where flashing red warning lights are used as required by s. 346.48 (2).

      (b)    Such standing vehicle must be capable of being seen by operators of other vehicles from a distance of 500 feet in each direction along such highway.

   (2)   This section also applies to vehicles or equipment used in highway maintenance or construction work unless the nature of the work is such as to require the stopping or standing of the vehicle or equipment on the roadway.