Human factors, i.e. the capabilities and limitations of road users, are key to safety performance. Chapter 2 of the AASHTO Highway Safety Manual (HSM) provides an excellent summary of human factors. A companion to the HSM, NCRHP 600 report, commonly referred to as the Human Factors Guideline, is available for free from TRB (http://www.trb.org/main/blurbs/167909.aspx).
In 2018, the Primer on the Joint Use of the Highway Safety Manual (HSM) and the Human Factors Guidelines (HFG) for Road Systems was released, adding valuable insights into the joint use of these two guides. (PDF)