Safety Data, Analysis, and Evaluation (ANB20)

1. MODELING SAFETY EFFECTS OF GEOMETRIC DESIGN CONSISTENCY
USING MIXED EFFECTS NEGATIVE BINOMIAL REGRESSION
RESEARCH OBJECTIVES METHODOLOGY
FIXED-LENGTH SEGMENTS
Transportation Research Board
2015 Annual Meeting
Variable Average Value Max. Min. Std. Dev.
COMPARING SPFs
TAKEAWAYS
BACKGROUND
DATA ACQUISITION
Develop fixed-length segments of 2.5 miles from HSIS data
Identify Sites with Potential (SWiPs) for safety improvements
SPF using a combination of typical and geometric consistency parameters
Conduct EB adjustments on crash data using MENB modified formula for overdisperison
SPF with typical roadway parameters SPF with geometric consistency parameters
Disparity of SWiPs identified between models will provide insights on the effectiveness
of these alternative design consistency measures in classifying potential unsafe roads.
*All parameters significant to the 95% level
To reduce heteroskedacity and improve estimation efficiency of regression analysis, fixed-length
segments of 2.5 miles were developed. Over 90 statistics (both typical and geometric design consistency)
were developed for each segment. A summary of the 8,969 segments is shown below:
# of Horizontal Curves
Average Radius (ft.)
Average Change in Radius (ft.)
Average Perceived Radius (ft.)
Average Change in Perceived Radius (ft.)
# of Intersections
AADT
Left Shoulder Width (ft.)
# of Left Shoulder Changes
Average Lane Width (ft.)
Average Grade (%)
Average Change in Grade (%)
# of Grade Changes
# of Accidents
5.76
2723.04
1367.16
2736.38
1393.99
3.46
2976.34
4.58
2.07
11.57
1.80
1.93
12.93
3.19
46
50000
32470
51990.42
35009.47
48
25505
10
29
19.23
8.64
11.50
90
61
0
0
0
0
0
0
16
0
0
8
0
0
0
0
5.426
3212.35
2281.23
3231.52
2298.44
3.89
3089.08
2.28
2.56
0.90
1.20
1.24
9.28
4.32
Develop and assess the use of direct geometric design
consistency parameters in safety performance functions (SPFs).
Previous research has attempted to model design consistency using parameters, such as:
However, many of these measures are not readily available to practitioners
and model design inconsistencies through indirect means.
Roadway inventory, geometric
alignment data, and 5 years of
crash data were obtained
through the Highway Safety
Information System (HSIS)
database for the state
of Washington.
Andy Butsick - Kittelson & Associates, Inc. - abutsick@kittelson.com
Paul Jovanis, PE, PhD - The Pennsylvania Sate University - ppj2@engr.psu.edu
Jonathan Wood - The Pennsylvania State University - jsw277@psu.edu
Speed Differences
Driver Workload
Vehicle Stability
Alignment Indices
Perceived Radius
Design inconsistency can be defined as:
Test the versatility of previously developed consistency measures
(alignment indices and driver-perceived radii) by incorporating them
in SPFs with other roadway parameters.
By doing so, this research hopes to model the true underlying conditions that
violate design consistency principles and provide practitioners with a
methodology for easily incorporating design consistency into their safety practices.
The presence of a geometric feature or combination of adjacent
features that violate driver expectancy, which in turn, may
surprise drivers and make them drive in an unsafe manner.{ }
Comprised of
over
4,900 miles
of two-lane
rural roadway
for use in analysis.
Disparity of SWiPs identified by a traditional SPF and one incorporating geometric
design consistency parameters can help elucidate potentially unsafe roadway
segments by drawing out the true underlying geometric inconsistencies.
Direct geometric design consistency parameters (e.g., max change in radius) offer a
practical means for safety practitioners to more readily incorporate design consistency
into predictive safety modeling over traditional design consistency parameters
(e.g. driver workload or speed differences).
PARAMETERS*
TYPICAL PARAMETER SPF
EFFECT ON
CRASHES
In(AADT)
# of Intersections
Avg. Radius
Avg. Curve Angle
Avg. Left Shoulder Width
Avg Lane Width
Avg Grade
PARAMETERS*
DESIGN CONSISTENCY SPF
EFFECT ON
CRASHES
In(AADT)
# of Intersections
Avg. Radius
Max. Change in Curve Angle
Avg. Degree of Curvature
Avg. Change in Curvature Degree
Avg. Left Shoulder Width
Max. Change in Left Shoulder Width
# Grade Changes
Typical SPF
Design Consistency SPF
TOP 220 (10% of segments) SWiPs Identified
40 (19%) unique sites identified by each SPF
Unique sites identified by the typical SPF may be the true SWiPs as the design inconsistencies
in these segments decrease their potential for improvement in the alternative SPF.