Bangkok to Khao Yai (1 Viewer)

For any of you who've been to Khao Yai, without your own transport, which was the best way of getting there from Pak Chong? I know I can get the bus from BK to Pak Chong, but once in Pak Chong, how would I get to the park HQ? More importantly, getting there is one thing, but how frequent is traffic back to the park entrance or Pak Chong? I don't want to get stuck for hours on end waiting for transport out of there.

I am planning to go there on Monday, until Wednesday.

Thanks