Any bus used primarily for the transportation of workers only and any bus operated by a church may bear upon the front and rear thereof a plainly visible sign, either painted or affixed on the body of the bus proper, or attached securely to the bus, containing the words "workers bus" or "church bus," respectively, in letters not less than eight inches in height. Any bus used primarily for the transportation of workers only and any bus operated by a church and bearing signs in that manner may lawfully stop upon the paved portion of any highway or street where there is no loading zone or pull-off adjacent to the highway or street to load or discharge persons: Provided, That such bus shall be equipped with warning lamps permitted under subsection (d), section nineteen, article fifteen of this chapter, and shall use such warning lamps when stopped on the highway or decreasing speed in order to stop, in order to warn the operators of other vehicles of a possible traffic hazard.
