RSAP is an encroachment-based computer software tool for cost-effectiveness evaluation of roadside safety improvements originally developed under NCHRP Project 22-9(1).  Subsequently, some improvements were made, bugs corrected and patches installed under NCHRP Project 22-9(2).  A third NCHRP project, 22-9(3) was initiated but never completed.  Various releases of RSAP have been distributed with the AASHTO Roadside Design Guide (RDG) since the 2002 edition.  This version of RSAP (i.e., RSAPv3) was developed under NCHRP Project 22-27 which was completed in 2012.

The analytical model behind the encroachment-based approach uses a series of conditionally independent probabilities representing vehicle roadside encroachment events, the conditional probability of a crash given a roadside encroachment has occurred, the probable severity of crashes that are likely to occur and the expected benefit cost ratios of various roadside design alternatives. Based on the sequential nature of these conditional probabilities and the assumption that they are independent, the RSAPv3 is basically structured into the following four modules, similar to its predecessors:

Encroachment module,

Crash prediction module,

Severity prediction module and

Benefit-cost module.