Final answer:
The exact speed requirement when passing snow plows on the highway can vary by jurisdiction, so local traffic regulations should be consulted for the correct procedure.
Step-by-step explanation:
When passing snow plows on the highway with warning lights on, drivers must adhere to specific safety guidelines, such as reducing their speed. However, without a specific jurisdiction or region mentioned, it's not possible to provide a definitive answer to this question as the rules can vary. In some places, regulations may require drivers to slow down to 20 or 30 km/h below the posted speed limit, or they may dictate slowing down to the minimum speed limit to ensure safety for all road users, including the snow plow operators. For the exact rule, one should consult the local traffic regulations or highway safety codes.