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for which crash data were collected, the data source, and the desirable for each of the roadway types. Table 3-10 lists the
roadway referencing systems. Table 3-8 defines the crash roadway data files and their sources. The research team
types used in the safety analysis. reviewed the data files to determine any missing data vari-
Extensive examination of the data was undertaken. The iter- ables in the recorded databases or hard copies. For every
ative process enabled the research team to improve and over- missing data variable deemed critical for comprehensive
come most of the issues found in the databases. Some of these safety evaluation of PRPM installations, means to collect
issues are described here. For example, during 1996, the loca- the information were explored, and whenever at all feasi-
tion referencing system in Illinois changed. Before 1996, the ble, those variables were collected by the members of the
reference for a route would restart at mile point zero each time research team. Some examples are described next.
it crossed a county boundary. After 1996, the reference for a The data received for two-lane treatment and reference
route would continue through the county boundary without group sites in New York and New Jersey contained no infor-
restarting at mile point zero. This change in the referencing mation on horizontal alignment (e.g., curve location, curve
system was reflected in the crash data files. To compute cor- radius, and curve length). This information for horizontal
rect and comparable before-and-after crash totals for the dif- curves for New York and New Jersey was obtained from
ferent PRPM roadway sections, the mile point data in the post- individual roadway design drawings, from New York DOT's
1996 crash and roadway attribute files for each PRPM route headquarters in Albany, and from New Jersey DOT's head-
were aligned with the data of the pre-1996 referencing system. quarters in Trenton.
Another example of data preparation is the assessment of Information on terrain type for New Jersey was obtained
crash counts on freeways and expressways in Wisconsin. This from video-log recordings. Information on terrain type for
data set revealed that there was a disproportionately greater Illinois was collected during field visits to District 8.
number of crashes recorded involving vehicles traveling north Tables 3-11, 3-12, and 3-13 provide a summary of the road-
and east than involving vehicles traveling south and west. way data collected at two-lane treatment sites for lane widths,
This observed anomaly is likely the result of crashes being degree of curvature, and terrain type, respectively. Tables 3-14
miscoded. Therefore, the safety analyses for freeways and through 3-19 provide a summary of the roadway data collected
expressways in Wisconsin considered the different travel for lane widths, shoulder widths, and environment types at
ways together. four-lane freeway and four-lane expressway treatment sites.
Non-intersection-related crashes were extracted from the
databases for the safety analysis of PRPM installations along
road segments. The daytime and nighttime crashes were 3.2.5 Traffic Volume Data
defined on the basis of the sunset and sunrise times received
from a national source of such times for different months of The variables required at treatment sites, reference group
the year for each state (38). sites, and comparison group sites were
· AADT volumes,
3.2.4 Roadway Attribute Data · Percentage of annual average nighttime traffic volumes,
and
Table 3-9 lists the roadway variables at PRPM and refer- · Percentage of heavy vehicles.
ence group sites according to their importance for the evalu-
ation analysis. The variables were classified as critical or (text continued p. 31)
TABLE 3-8 Crash type definitions
Crash Type Definition
Total All crashes reported and entered in the database
Fatal and injury Crashes that resulted in fatal or nonfatal injuries
Daytime Crashes that occurred between sunrise and sunset
Nighttime Crashes that occurred between sunset and sunrise
Dry Crashes that occurred on "road surface condition" reported as "dry"
Wet Crashes that occurred on "road surface condition" reported as snow, wet,
ice, or any other nondry conditions
Wet-nighttime Crashes that occurred "road surface condition" reported as snow, wet,
ice, or any other nondry conditions between sunset and sunrise
Guidance-related Crashes with reported "impact type" as run-off-road, head-on, and
sideswipe for fatal, injury, and property-damage-only combined
Head-on Crashes with reported "impact type" as head-on for fatal, injury, and
property-damage-only combined
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TABLE 3-9 Critical and desirable roadway variables
Variables Two- Four-lane Four-lane
lane expressway freeway
Type of location (e.g., curve or tangent)
Terrain type (flat, rolling, mountainous)
Type of access control
Roadway width
Number of lanes
Lane width
Median type (e.g., raised, painted, and no median) NR
Left and right shoulder types (e.g., surfaced and
gravel)
Horizontal alignment (e.g., degree of curve)
Vertical alignment (e.g., grade and vertical
curvature)
Median width NR
Left and right shoulder widths
Design speed
Speed limit
85th percentile speed
= Critical variables.
= Desirable variables.
NR = Variables not relevant (two-lane roadways do not have medians).
TABLE 3-10 Roadway data files and their sources
State Data Files Received Source
Illinois Sufficiency files for 1991, 1992, 1994, and Highway Safety
1997 to 2000 Information System
(HSIS)
Missouri Roadway inventory Missouri DOT
Rumble strip inventory
Median inventory
New Jersey Straight line diagrams (1994 to 2001) New Jersey DOT
New York Sufficiency files for 1991 to 2001 New York DOT
Pennsylvania Roadway inventory Pennsylvania DOT
Guiderail inventory
Shoulder inventory
Alignment data
Wisconsin Highway log Wisconsin DOT
TABLE 3-11 Summary information: lane widths at two-lane treatment sites
State % of total % of total % of total % of total Minimum Maximum Average
roadway roadway roadway roadway lane lane width lane
length for length for lane length for lane length for width (ft) (ft) width
lane widths widths lane (ft)
widths > 10 ft and > 11 ft and widths
10 ft 12 ft >12 ft
11 ft
Illinois 5.0 30.2 54.3 10.6 8.0 22.0 11.6
New Jersey 44.0 6.5 46.6 2.8 10.0 25.0 11.2
New York 2.4 22.8 72.2 2.6 10.0 25.0 11.8
Pennsylvania 13.7 32.3 45.6 8.4 8.0 32.0 11.8
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TABLE 3-12 Summary information: degree of curvature
at two-lane treatment sites
State % of total % of total % of total Minimum Maximum Average
roadway roadway roadway DOC DOC DOC
length length length
when when when
DOC = 0 DOC DOC >
3.5 3.5
Illinois 97.4 0.8 1.8 0.0 114.9 0.13
New Jersey 87.1 9.0 3.9 0.0 9.99 0.36
New York 68.5 18.7 12.8 0.0 76.4 4.04
Pennsylvania 50.0 35.6 14.4 0.0 68.24 1.61
DOC = degree of curvature.
TABLE 3-13 Summary information: terrain type
at two-lane treatment sites
State % of total % of total % of total
roadway roadway roadway
length on flat length on length on
terrain rolling terrain mountainous
terrain
Illinois 81.9 9.1 0.0
New Jersey 61.1 38.9 0.0
New York 61.5 38.5 0.0
Pennsylvania 85.0 15.0 0.0
TABLE 3-14 Summary information: lane widths
at four-lane freeway treatment sites
State % of total % of total % of total Minimum lane Maximum Average
roadway roadway roadway width (ft) lane width lane width
length length length (ft) (ft)
when lane when lane when lane
width width width
12 ft
Missouri 0.1 98.7 1.2 11.0 18.0 12.0
New York 0.5 98.7 0.8 9.0 17.0 12.0
Pennsylvania 1.9 94.2 3.5 10.0 25.5 12.2
Wisconsin 0.0 99.7 0.3 12.0 18.0 12.0
TABLE 3-15 Summary information: shoulder widths at four-lane freeway treatment sites
State % of % of total % of total % of total % of Minimum Maximum Average
total roadway roadway roadway total shoulder width shoulder width shoulder width
roadway length when length when length when roadway (ft) (ft) (ft)
length shoulder shoulder shoulder length
when width width width when
shoulder > 4 ft and > 6 ft and > 8 ft and shoulder
width 6 ft 8 ft 10 ft width
4 ft > 10 ft
Missouri 8.2 2.9 2.2 86.7 0.0 3.0 10.0 9.3
New York 0.2 0.1 2.0 56.8 40.9 0.0 12.0 10.8
Pennsylvania 45.7 3.0 6.3 43.1 2.0 0.0 12.0 6.3
Wisconsin 0.3 0.4 63.1 33.7 2.5 0.0 13.0 8.5
