Last time I looked I got them from the Berks county planning commission website... I just checked, it now forwards me to a penndot site, looks like exactly what you're looking for:
http://www.dot7.state.pa.us/itms/open.aspx
If it is a state maintained highway contact your state highway and transportation dept. You can also call the traffic division in your city. Traffic counts are usually conducted at specific points along a highway. In my state they use the same spots and they each have a number and they can give the counts for that spot for several years back