IHSDM Data Requirements:

The data requirements by module are provided in this section.

IHSDM Data Requirements – Policy Review Module

• General data: terrain, functional class, design speed, AADT, DHV, and Emax.
• Complete horizontal and vertical alignment data.
• Cross section: cross slope, auxiliary lane, thru lane, lane offset, TWLTL, median, curve widening, shoulder definition, shoulder rounding, bridge elements, and surface type.
• Roadside: bike facility, roadside slope, obstruction offset.
• Policy specific review: decision sight distance maneuver.

IHSDM Data Requirements – Crash Prediction Module

• Evaluation bounds (beginning and end station) and evaluation period.
• Highway segment geometric and traffic control data: lane width, shoulder width/type, driveway density, roadside hazard rating (RHR), horizontal curve data, grade, passing lanes, and center TWLTLs.
• Intersection geometric and traffic control data: number of intersection legs, type of traffic control, approach (leg) type(i.e., major leg/minor leg), intersection skew angles, number of major-road approaches (legs) with exclusive left-turn lanes, number of major-road approaches (legs) with exclusive right-turn lanes, number of intersection quadrants with limited intersection sight distance.
• Highway segment traffic volume data: ADT for the entire evaluation segment and time period. If ADT data are missing for any years or segments, the module will interpolate missing ADT data.
• Intersection traffic volume data: ADT for each intersection for the entire evaluation segment and time period.
• Crash history data including the year of crash occurrence, severity level, location, relationship to intersection, and intersection station.

IHSDM Data Requirements – Design Consistency Module

• General elements: design, posted, and desired speeds.
• Complete horizontal and vertical alignment data.
• Roadside hazard rating.
• Vehicle type.
• Speeds at evaluation start and end stations.

IHSDM Data Requirements – Traffic Analysis Module

• Posted speed.
• Complete horizontal and vertical alignment data.
• Cross section data: cross slope, auxiliary lane (climbing and passing if present), thru lane, lane offset, median, and definition.
• Roadside obstruction offset.
• Traffic analysis data: report location, crawl region, available passing sight distance, reduced speed zone, and no passing zone.
• Evaluation bounds.
• Traffic flow (flow rate, entering platoons %, percent truck, percent RV).
• Upstream alignment (increasing stations, decreasing stations).
• Desired speed (mean, standard deviation for cars, trucks, and RVs).
• Simulation control (warm-up period, test period, random number seeds).

IHSDM Data Requirements – Driver Vehicle Module

• Posted speed.
• Complete horizontal and vertical alignment data.
• Cross section elements: through lane, auxiliary lane (passing, climbing, turn, two-way left-turn lane), shoulder section, lane offset, curve widening, and cross slope.
• Roadside obstruction offset.
• Evaluation bounds.
• Simulation direction (increasing or decreasing).
• Evaluation mode (deterministic or stochastic).
• If stochastic mode then the number of trials and seed.
• Driver type (nominal or aggressive).
• Path decision (center or cut-curve).
• Vehicle type (passenger car or truck).
• Simulation settings (report interval, integration time interval, free speed, obeying speed limit, road familiarity).
• Summary report interval.
• Create detailed output file.

IHSDM Data Requirements – Intersection Review Module

• General elements: design speed, 85th percentile speed, AADT, DHV, PHV, and functional classification.
• Complete horizontal and vertical alignment data.
• Cross-section elements: thru Lane, auxiliary lane (if present), cross slope, and lane offset.
• Intersection elements: default values are assigned when defining the intersection, but user may change the values as appropriate.
• Intersection attributes: intersection name, number of legs, construction type, traffic control, intersecting highway dataset, station at point of intersection (of the intersecting highway), and if signalized, night flashing policy.
• Intersection legs general information: relative heading, and classification (major/minor).
• Traffic attributes on intersection legs: traffic control position, turn restriction, stop line offset, right turn on red allowed, channelization, traffic turn data, and curb type.

IHSDM Analysis Methodology

The output from IHSDM can be used to identify potential issues for detailed field investigation by the RSA team and can help point to potential measures that may mitigate safety issues identified as part of the RSA process. IHSDM can help focus an RSA team's efforts to maximize the input from the multidisciplinary team. The analysis methods used for each of the modules are as follows.

IHSDM Analysis Methodology – Policy Review Module

The evaluation report for this module contains tables comparing user-specified elements against user-specified policies along with a graphical display of the stopping site distance and passing sight distance checks. The user may select from 17 policy checks, organized by category as follows:

• Cross Section,
• Horizontal Alignment,
• Vertical Alignment, and
• Sight Distance.

For this effort the AASHTO 2011 policy provided in IHSDM was used to perform selected checks. The checks selected were:

• Length of curve
• Radius of curve
• Stopping Sight Distance
• Passing Sight Distance

IHSDM Analysis Methodology – Crash Prediction Module

The two-lane rural roads predictive method was utilized with site-specific crash data, which utilizes the Empirical Bayes (EB) procedure. This procedure combines expected crash frequencies (estimated using the safety performance functions and crash modification factors) with site-specific crash history data. The advantage of using the EB procedure is to improve the accuracy of estimates for an individual location by factoring in the actual crash history of the location being evaluated. In theory, when enough years of crash history data are available, it would be more accurate to appropriately combine estimates from the SPFs and CMFs with site-specific crash history data than to rely on either the model estimates or the site-specific data alone. The EB procedure determines the statistically appropriate weighting of estimates from the SPFs and CMFs and site-specific crash history data.

The predictive model for Rural Two-Lane, Two-Way Roadway Segments is described in Chapter 10 of the HSM. The CPM report provides tables with the highway and intersection data provided to conduct the evaluation, along with plotted expected crash frequencies and rates using the HSM predictive method.

Crash prediction models for roadway segments of Rural Two-Lane highways are composed of two analytical components: safety performance functions (SPFs), or baseline models, and crash modification factors (CMFs). There are also calibration factors that adjust the predictions to a particular jurisdiction or geographical area. The general form of the crash prediction models for roadway segments is:

Nrs = Nspf-rs x Cr x (CMF1r...CMFnr)

where:
Nrs= predicted number of crashes for roadway segment per year;
Nspf-rs = predicted number of roadway segment crashes per year for nominal or baseline conditions;
Cr = Calibration factor for roadway segments developed for use for a particular jurisdiction or geographical area; (See the Highway Safety Manual, Appendix to Part C for details on calibration of the crash prediction models.)
CMFnr = crash modification factors for roadway segments.

The IHSDM approach to applying the predictive crash model relies on dividing the subject roadway into homogeneous segments, referred to as "sites." These segments are defined based on the locations of intersections, horizontal curves, vertical curves, and additional lanes or changes in average daily traffic or various roadway geometry attributes. The subject roadway is thus divided into a set of contiguous segments, or sites. The predictive crash model is used to calculate a crash estimate for each segment.

Crash estimates for each segment are dependent on the Crash Modification Factors (CMF) applied to the base model. A crash modification factor (CMF) is a multiplicative factor used to compute the expected number of crashes after implementing a given countermeasure at a specific site. CMFs are used to adjust the SPF estimate of predicted average crash frequency for the effect of individual geometric design and traffic control features, as stated in the crash prediction formula. CMFs in the range greater than 1.00 will increase the average crash frequency estimate, while CMF values below 1.00 will reduce the average crash frequency estimate. The IHSDM Predictive Crash methodology utilizes the following CMFs:

• Lane Width (CMF1r)
• Shoulder Width and Type on Highway Segments (CMF2r)
• Horizontal Curves (CMF3r)
• Superelevation (CMF4r)
• Grade (CMF5r)
• Driveway Density (CMF6r)
• Centerline Rumble Strips (CMF7r)
• Passing Lane (CMF8r)
• Two-Way Left-Turn Lane (CMF9r)
• Roadside Design, Roadside Hazard Rating (CMF10r)
• Lighting (CMF11r)
• Automated Speed Enforcement (CMF12r)

As part of this project, the IHSDM predictive crash model was used to evaluate crash potential for each of the RSA corridors. For the roadway segments evaluated on these corridors the differences in the predicted segment crash rates were primarily influenced by two specific CMFs: Horizontal Curves (CMF3r) and Grade (CMF5r). This finding suggests that the roadways’ curvature and grade contribute to a disproportionate number of crashes on these corridors. The following sections summarize the CMF calculations for horizontal curves and grade.

Horizontal Curves (CMF3r)

The base condition for horizontal alignment is a tangent roadway segment. The CMF for horizontal curvature is in the form of an equation and yields a factor. The CMF for length, radius, and presence or absence of spiral transitions on horizontal curves is determined using the following equation:

CMF3r = (1.55Lc + 80.2/R – 0.12S)/1.55Lc (3.7)

where:
CMF3r = Crash Modification Factor for the effect of horizontal alignment on total crashes; Lc = Length of horizontal curve (miles) which includes spiral transitions, if present;
R = Radius of curvature (feet);
S = 1 if spiral transition curve is present; 0 if spiral transition curve is not present; 0.5 if a spiral transition curve
is present at one but not both ends of the horizontal curve.

Some roadway segments being analyzed may include only a portion of a horizontal curve. In this case, Lc represents the length of the entire horizontal curve, including portions of the horizontal curve that may lie outside the roadway segment of interest.

CMF values are computed separately for each horizontal curve in a horizontal curve set (compound curves). For each individual curve, the value of Lc used in the equation is the total length of the compound curve set and the value of R is the radius of the individual curve. If the value of CMF3r is less than 1.00, the value of CMF3r is set equal to 1.00.

Grade (CMF5r)

The base condition for the CMF for grade is a level roadway (0% grade). The following table (HSM Table 10-11) presents the CMF for grades. This CMF is applied to each individual grade segment on the roadway being evaluated without respect to the sign of the grade. The grade factors are applied to the entire grade from one point of vertical intersection (PVI) to the next (i.e., there is no special account taken of vertical curves).

Table 8: CMF for Grade for Roadway Segments (HSM, Table 10-11)

Calibration factor was not applied to the RSA corridors selected for this study due to the availability of data to calibrate the models.

IHSDM Analysis Methodology – Design Consistency Module

1. This module evaluates operating speed consistency as follows:
2. Calculates the expected difference between estimated 85th percentile speeds and the design speed.

Calculates the expected reduction in estimated 85th percentile speeds from an approach tangent to its succeeding horizontal curve.

These measures are based on a speed-profile generated for a selected alignment. The steps in developing the speed profile are as follows:

Step 1 – Determine Preferred Speed

IHSDM determines the preferred speed on each horizontal highway element (i.e. horizontal tangents and curves) using different models.

Step 1a – Check Posted Speed
The Posted Speed is checked for the highway section. Different sets of models are used for higher speed highways vs. lower speed highways.

Step 1b – Determine Preferred Speed for Each Tangent
For higher speed highways, a desired speed is selected on tangents. For lower speed highways, the DCM predicts the preferred speed for each horizontal tangent using empirical speed prediction equations, based on data collected from tangent sites around the U.S. For tangents less than 150 ft in length, tangent Speed is a function of Posted Speed and Preceding Curve Radius. For tangents 150 ft or greater in length, tangent speed is a function of Posted Speed, Roadside Hazard Rating, and tangent Length.

Step 1c – Predict Preferred Speed for Each Curve
DCM predicts the preferred speed for each horizontal curve using empirical speed prediction equations, based on data collected from curve sites around the U.S. Speed is assumed to remain constant throughout a horizontal curve for both higher speed and lower speed highways.

For higher speed highways, curve speed is a function of curve radius and is affected by the vertical alignment. For lower speed highways, curve speed is a function of curve radius only.

Step 2 – Adjust Speeds for Acceleration and Deceleration

Speeds are adjusted using acceleration and deceleration characteristics of the evaluation vehicle selected for the analysis.

Step 3 – Preferred Grade-Limited Speeds Using TWOPAS Equations

Grade-limited speeds are predicted using TWOPAS equations. A second speed profile is generated, based on the vertical alignment and the evaluation vehicle selected for the analysis.

Step 4 – Select Lowest Speeds for Each Location

The lowest speed is selected for each location. The speed profiles from Steps 2 and 3 are compared and the lowest speed selected at each point along the alignment to create a final speed profile.

The DCM report contains a graph of the evaluation results on the speed profile along with a variety of geometric characteristics for the alignment. The report contains both graphical and tabular summaries of the DCM results.