Officials said Tuesday that the limit would remain in effect "until the weather improves."
The state transportation department also imposed a 45 mph speed limit on some roads due to the winter storm.
The National Weather Service says the storm brought some rare "thundersnow" - heavy snow with embedded thunder - to western Pennsylvania.
Forecasters say 6.5 inches of snow has been reported in Butler County, 6 inches in Lawrence County and more than 5 inches in Somerset County, with 3 inches at Philadelphia International Airport.
