Roadside Safety Analysis Program (RSAP) Update (2022)

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17 PROGRAM OVERVIEW RSAP is an encroachment-based computer software tool for cost-effectiveness evaluation of roadside safety improvements originally developed under NCHRP Project 22-9(1) [Mak03]. Subsequently, some improvements were made, bugs corrected and patches installed under NCHRP Project 22-9(2) [Mak03]. 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 [AASHTO02]. This research effort updated RSAP to the third version (i.e., RSAPv3). The encroachment method used in RSAPv3 and previous versions of RSAP is conceptually straight forward, however estimating the three conditional probabilities at the heart of the method can be very difficult and computationally tedious. Each of these conditional probabilities must be developed based on some type of statistical model which in turn is either based on observed encroachment and crash data or assumptions about the way crashes happened in the field. Since the computations can be complicated, a computer program has seemed to be the most useful way to implement the encroachment-based approach in roadside safety analysis. 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. BACKGROUND Prior to the introduction of RSAPv3, RSAP version 2.0 and the ROADSIDE program have been included in the AASHTO Roadside Design Guide since 1989. RSAP version 2.0 was intended to replace and improve upon the ROADSIDE program. [AASHTO89; AASHTO96] RSAP version 2.0 provided many procedural and algorithmic improvements over its predecessor, ROADSIDE as summarized in Table 1. RSAP version 2.0 also provided a graphical interface making it much easier to use than the ROADSIDE program. In the roadside safety community, RSAP version 2.0 was widely considered as the state-of-the-art safety evaluation software tool at the time. In order to address the more complex encroachment relationships and compensate for lack of available data, RSAP version 2.0 adopted the Monte Carlo simulation approach to generate the solution, as opposed to the deterministic approach used in the ROADSIDE program.

18 Table 1. Comparison of RSAP and ROADSIDE. [after Mak03] Parameter ROADSIDE RSAP version 2.0 Encroachment Rate A constant of 0.0003 encroachments per km per year per vehicle per day Cooper encroachment data, adjusted for encroachments with lateral extent of encroachment less than or equal to 4m Vehicle Type Single vehicle type 12 vehicle types based on nominal percent truck Encroachment Speed Function of design speed Same as impact speed Encroachment Angle Average angle based on point- mass model Same as impact angle Vehicle Orientation No Based on “real-world” crash data Lateral Extent of Encroachment Assumes 3.7 m/sec/sec (0.4 g) deceleration rate and sine curve density function for steer back Cooper encroachment data, adjusted for encroachments with lateral extent of encroachment less than or equal to 4m Shielding of One Hazard by Another No Yes Multiple Hazards Each hazard has to be analyzed individually and the crash costs summed manually Yes Effect of Barrier Protection All impacts with hazard shielded by barrier eliminated, regardless of barrier length Vehicles encroaching upstream of barrier could impact hazard shielded by barrier Rollover Crashes No Rollovers initiated by hitting fixed objects Cross Median Crashes No Not Really (Can potentially “trick” the program to emulate the event, but not recommended) Impact Speed = Encroachment speed - speed loss with 3.7 m/sec/sec (0.4 g) deceleration rate Based on “real-world” crash data Impact Angle Same as encroachment angle Based on “real-world” crash data Severity (SI) Average values only Function of impact speed Incremental B/C Ratios for Multiple Alternatives Have to be calculated manually Yes Solution Method Deterministic Stochastic, using the Monte Carlo simulation technique RSAP version 2.0 consisted of two relatively independent, but interoperable, programs: the Main Analysis Program and User Interface Program. The Main Analysis Program is written in the FORTRAN programming language and it contains the procedures, models, formulas, and data for cost-effectiveness analysis. The User Interface Program is written in the C++ programming language and provides a sequence of simple menus for users to specify input data and review analysis results in a Microsoft Windows environment.

19 The two most significant documents related to the RSAP version 2.0 are: • Mak, K. K. and Sicking, D. L., “Roadside Safety Analysis Program (RSAP) – Engineer’s Manual,” NCHRP Report No. 492, National Cooperative Highway Research Program, Transportation Research Board, Washington, D. C., 2003.[Mak03] • Mak, K.K. and Sicking, D.L. “Roadside Safety Analysis Program (RSAP)— User’s Manual.” Prepared for NCHRP Project 22-9, National Cooperative Highway Research Program, Transportation Research Board, National Research Council, 2002.[Mak02] The RSAP version 2.0 Engineer’s Manual describes the inner workings of the cost- effectiveness analysis procedure and the various formulas, algorithms and data built into the procedure. The separate User’s Manual describes the User Interface Program, including the data input process and the output from the program. MODELING ENCROACHMENTS The encroachment module in RSAPv3 deals with the first key question in assessing the benefit-cost of a roadside safety design. Namely, how often do roadside encroachments occur by highway type, traffic volume, and highway geometric characteristics such as the number of lanes, horizontal curvature and grade? This question is addressed in RSAPv3 through the incorporation of base encroachment rates which quantify how often vehicles leave the traveled way (i.e., the marked lanes) and inadvertently or unintentionally enter the roadside. The base encroachment frequency is for a relatively straight, flat highway. Base encroachment frequencies are expressed as the number of vehicle roadside encroachments per unit length per year as a function of traffic volume. Adjustment factors are used to include the effects of horizontal curvature, grade, number of lanes, lane width, access density, and posted speed limit. RSAPv3 relies on the so-called Cooper encroachment data which were collected in the 1970s as described above. [Cooper80] Many changes have occurred in the interim (e.g., improved highway and roadside designs, higher speed limits, higher traffic volumes, and better vehicle safety equipment). Unfortunately, no newer or better encroachment data are available. Efforts to collect encroachment data with videotape surveillance and electronic monitoring system in the 1980s were not successful. There have also been exploratory efforts to estimate encroachment rates from police-level crash data and statistical modeling, some of which are still ongoing. MODELING CRASHES Recently completed and ongoing efforts to gather data on vehicle trajectories during an encroachment [Mak10; Gabler12] have been incorporated into RSAPv3 to model vehicle paths. This field gathered trajectories are superimposed on the user-entered roadside terrain and an assessment of all possible interaction of trajectories with user-entered hazards is made. The Engineer’s Manual (i.e., Appendix B) provides the necessary details to understand this process and to add new trajectory data. This approach allows RSAPv3 to use a completely deterministic approach rather than the Monte Carlo approach used in earlier versions of RSAP. Since the trajectories used in RSAPv3 were recorded based on real-world trajectories, the behaviors of typical drivers in typical vehicles are inherently incorporated. Where the