Safety Effects of Horizontal Curve and Grade Combinations on Rural Two-Lane Highways

CHAPTER 3—DATABASE DESCRIPTION

This chapter describes the database used in the analysis. The research was performed with HSIS data for State highways in Washington. This is the only data source that includes system-wide data on curve and grade geometry that can be linked to system-wide roadway characteristics, traffic volume, and crash data.

DATABASE DEVELOPMENT

Research began with a review of available databases that contain roadway data (including horizontal and vertical alignment), traffic volume data, and crash data in a format that could be linked by location, with a primary focus on available HSIS data. The only dataset found with sufficient detail concerning horizontal and vertical alignment was HSIS data for State highways in Washington.

Preliminary data processing was performed to identify the following types of roadways on the Washington State highway system (note that the abbreviations are used in table 2 through table 5):

• Rural two-lane highways (R2U).
• Rural four-lane undivided highways (R4U).
• Rural four-lane divided highways (nonfreeways) (R4D).
• Urban/suburban two-lane undivided arterials (U2U).
• Urban/suburban three-lane arterials with a center two-way left-turn lane (U3T).
• Urban/suburban four-lane undivided arterials (U4U).
• Urban/suburban four-lane divided arterials (nonfreeways) (U4D).
• Urban/suburban five-lane arterials with a center two-way left-turn lane (U5T).

Roadway segments with atypical features such as passing and climbing lanes as well as roadway segments with transitions between grades identified as angle points were eliminated from consideration. Angle points most likely represent crest or sag vertical curves that were too short or not well enough defined to be measured properly. Finally, a limited set of roadways with obvious data problems, such as successive vertical curves whose lengths appeared to overlap, were also eliminated from consideration. The descriptive statistics for the roadway data shown below are based on the roadway lengths that were retained for analysis after the initial screening.

Next, each roadway segment was classified into categories by its horizontal and vertical alignment. Horizontal alignment was classified as follows:

• Tangent roadways.
• Roadways on horizontal curves.

Vertical alignment was classified as follows:

• Level roadways.
• Straight grades (constant percent grade of 1 percent or more).
• Type 1 crest vertical curves.
• Type 2 crest vertical curves.
• Type 1 sag vertical curves.
• Type 2 sag vertical curves.

Figure 1 illustrates the distinction between types 1 and 2 crest and sag vertical curves. Every roadway segment was defined by its horizontal alignment, vertical alignment, and combination of horizontal and vertical alignments. Since there were two horizontal alignment categories and six vertical alignment categories, there were a total of 12 possible combinations of horizontal and vertical alignments. Where horizontal and vertical curves overlap, their beginnings and ends may not coincide; therefore, a new roadway segment began at any point where the horizontal or vertical alignment changed. Thus, some segments might include all of a horizontal or vertical curve, while others might include only part of a horizontal or vertical curve. The length of every roadway segment was determined for use in the analysis as well as the length of any horizontal curve that was wholly or partially within the segment and the length of any vertical curve (L_VC) that was wholly or partially within the segment. Additionally, each horizontal curve was characterized by its radius. No data on the superelevation of horizontal curves were available for analysis. Each straight grade was characterized by its percent grade (G). Each vertical curve was characterized by its approach grade (G₁) and departure grade (G₂), its algebraic difference in grade (A = abs(G₁ – G₂)) [A = abs(G₁ – G₂ )]*, and the ratio of its length to its algebraic difference in grade (K = L_VC/A).

*Modified on November 16, 2014

Crash data for a 6 year period (2003 to 2008) were obtained and used in the analysis. Each crash was assigned to a particular roadway segment, with particular horizontal and vertical alignment based on its assigned milepost location. Since the results of this research are intended for use in the roadway segment procedures of the AASHTO HSM, only nonintersection crashes were considered. Nonintersection crashes are those that did not occur at an intersection and were not classified by the investigating officer or data coder as related to the operation of an intersection. The traffic volume for each roadway segment was determined from available traffic volume data.

Some of the roadway segments included in the analysis may have contained minor intersections, but crashes occurring at or related to those intersections have been excluded from the analysis. No major intersections were included within a roadway segment because a change in traffic volume along the roadway would have led to the creation of a roadway segment boundary.

Driveway crashes are considered roadway segment crashes. As a result, they were included in the analysis. The amount of driveway access within segments may have varied, but all of the segments were located in areas classified as rural.

DESCRIPTIVE STATISTICS

The following discussion presents descriptive statistics for the roadway sections available for analysis including roadway length (miles), exposure (million vehicle miles traveled in the 6 year period (MVMT)) [million vehicle miles traveled in the 6-year period (MVMT)]*, crash frequencies, and crash rates per MVMT for specific combinations of horizontal and vertical alignment and roadway type. Roadway length, exposure, crash frequency, and crash rate are presented in table 2 through table 5, respectively.

*Modified on November 16, 2014

Of the 6,944 mi of roadway in the entire Washington HSIS database, 4,785 mi (69 percent) are on rural two-lane highways. Of these, 3,457 mi were used for analysis. Rural two-lane highways with passing or climbing lanes and segments with missing or obviously incorrect alignment data (e.g., overlapping curves) were excluded from the study. Therefore, based on data availability, the analysis of the safety effects of horizontal curve and grade combinations in this report focused on rural two-lane highways only.

Table 2. Roadway length (mi) in available data from Washington HSIS database.

Table 3. Exposure (MVMT) in available data from Washington HSIS database (2003–2008).

Table 4. Fatal and injury (FI), property damage only (PDO), and total 6 year crash frequencies in available data from Washington HSIS database.

Table 5. FI, PDO, and total crash rates per MVMT in available data from Washington HSIS database.

The majority of rural two-lane roadway segments included in the analysis experienced no crashes in the 6 year period. Table 6 shows the percentage of roadway segments with no crashes and with one or more crashes for each roadway and severity type. The data clearly show the highly skewed crash distribution for all severity types and all roadway types. The high percentages of roadway segments without crashes (78 to 89 percent for FI crashes and 72 to 84 percent for PDO crashes) provided a challenge in modeling crash frequencies.

Table 6. Percent of sections with and without crashes on rural two-lane highways in available data from Washington HSIS database (2003–2008).

VALUE RANGE OF ROADWAY CHARACTERISTICS

Prior to statistical modeling, the parameters of interest were assessed for extreme values (both high and low). This was done using a combination of plots of crash rates per MVMT versus selected parameters and distributions of the individual parameters. The following rules were implemented:

• Roadway segments less than 0.01 mi long were excluded from analysis (such short segments likely represent small overlaps in horizontal and vertical features and are unlikely to be useful analysis sections).
• For type 1 crest and type 1 sag vertical curves, segments where both initial (G₁) and final (G₂) grades were less than 1 percent in absolute value were excluded (such minor vertical curves are very close to being level).
• For type 2 crest and type 2 sag vertical curves, segments where A = abs(G₁ – G₂) was less than 1 percent were excluded (such minor vertical curves are very close to being straight grades).
• All records with K exceeding 1,000 were excluded (these were typically long, vertical curves with small grade changes and could be classified as straight grades, including level grades).
• All records with a curve radius exceeding 11,460 ft were excluded (these could be classified as tangents for all practical purposes).
• Horizontal curves with a radius less than 100 ft were included in the analysis, but the radius was set at 100 ft based on guidance in the AASHTO HSM.⁽¹⁾