South towards Tel Aviv
Walk east from the central station on Herzel St. until you reach the bridge going over the train tracks and road no. 2. There is a path or stairs going below the bridge. For the direction south, there's a trempiyada (Google street view).
North towards Haifa
For going north, you have to go over the bridge and take a right, it's less comfortable and few cars pass here going north but still possible (Google street view). Another possibility is to wait on the on-ramp from Netanya to highway 2 north. There isn't a proper place to stop, but if you're bold this should be better than the latter hitch point (Google street view).