Impact of Intersection Angle on Safety HSIS Annual Liaison Meeting David Harkey, Bo Lan, Daniel Carter, Raghavan Srinivasan, Anusha Patel Nujjetty May 15, 2013 Definition Intersection Angle Current Design Guidance Desire 90 degree intersections Minimum critical angle range of 60 to 75 degrees, depending on which policy/resource AASHTO 60 FHWA 75 ITE 60, 70, 75 Montana DOT 60 Illinois DOT 75 CalTrans - 75 What we know? Very few crash-based studies HSM recommendations from cross-sectional studies Expert panel recommendation for further work Visibility research suggest critical minimum angles greater than the current AASHTO policy Older persons and visually-impaired pedestrians may be negatively impacted at skewed intersections Research Objective Derive quantitative relationships between intersection angle and safety CMFs for intersection angle Assessment of critical minimum angle Revisions to design policies Safety is measured in terms of crashes Methodology Analysis approach Before-after evaluation (not feasible) Cross-sectional modeling Data mining Random forest Classification and regression trees Large sample of intersections Skewed and not skewed Range of intersection angles Data Acquisition Minnesota Intersection (skew attribute) Tee vs. Wye and Cross vs. Skew Crash, roadway, traffic Supplemental data Verification of skew Measured intersection angles Other potential contributing data elements Supplemental Data Collection Google Maps aerial & street view images geometric/land use features MN GIS base layer ArcGIS angles (COGO tool) Final Database Angle Classification Rural Intersections Urban Intersections Total No.% % Right Angle Skewed Total Angle Classification Rural Intersections Urban Intersections Total No.% % Right Angle Skewed Total leg 3-leg Model Development Negative binomial models State-of-the-Practice Highway Safety Manual safety prediction models Accounts for overdispersion (variance > mean) Modeling framework 3-leg vs 4-leg intersections Rural vs urban Two-lane vs multilane Total vs injury vs PDO Model Development Model forms Base models Flexible form models Interaction model All variables in final models are significant (p < 0.05) Model fit BIC selected as primary metric AIC selected as secondary metric Base Models Initial base model Intersection AADT Minor Road AADT Alternative base model Minor Road AADT / Intersection AADT Base models assume a log-linear relationship between crashes and angle improved fit Crash Rates Intersection Angle Flexible Form Models Model form that better replicated the raw data with respect to intersection angle 1+cos(angle) CMF Development CMFs derived from all models that included a significant intersection angle variable Regression coefficients ( ) applied to value of angle ( ) for all significant angle terms All CMF values normed to a nominal value of 1.0 for a 90 degree intersection CMF (Flex1, Flex2, and Base Models) CMF(base) = EXP( 1 ) CMF(flex1) = EXP( 1 ) x (1+cos )^ 2 + 3 CMF(flex2) = EXP( 1 ) x (1+cos )^ 2 4-Leg CMFs Results for skewed intersections < 40 degrees are based on a small sample of intersections 4-Leg Rural CMFs Results for skewed intersections < 40 degrees are based on a small sample of intersections 2-Lane HSM Comparison Multilane HSM Comparison Critical Angle Assessment Intersection Scenario & Collision Category 80 Degrees 75 Degrees 70 Degrees 65 Degrees 60 Degrees 55 Degrees All Sites, Total All Sites, Injury All Sites, PDO Rural Sites, Total Rural Sites, Injury Rural Sites, PDO Rural Two-Lane Sites, Total Rural Two-Lane Sites, Injury Rural Two-Lane Sites, PDO Rural Multilane Sites, Total Rural Multilane Sites, Injury Rural Multilane Sites, PDO Critical Angle Assessment Intersection Scenario & Collision Category 80 Degrees 75 Degrees 70 Degrees 65 Degrees 60 Degrees 55 Degrees All Sites, Total All Sites, Injury All Sites, PDO Rural Sites, Total Rural Sites, Injury Rural Sites, PDO Rural Two-Lane Sites, Total Rural Two-Lane Sites, Injury Rural Two-Lane Sites, PDO Rural Multilane Sites, Total Rural Multilane Sites, Injury Rural Multilane Sites, PDO CMF < 1.10 Critical Angle Assessment Intersection Scenario & Collision Category 80 Degrees 75 Degrees 70 Degrees 65 Degrees 60 Degrees 55 Degrees All Sites, Total All Sites, Injury All Sites, PDO Rural Sites, Total Rural Sites, Injury Rural Sites, PDO Rural Two-Lane Sites, Total Rural Two-Lane Sites, Injury Rural Two-Lane Sites, PDO Rural Multilane Sites, Total Rural Multilane Sites, Injury Rural Multilane Sites, PDO Implications of Results Intersection Angles with Highest Crash Reduction Potential Results for skewed intersections < 40 degrees for two-lane roads and < 50 degrees for multilane roads are based on a small sample of intersections Research Conclusions Safety Relationship Derivation of CMFs 14 CMFs 3-leg and 4-leg All, Rural, Two-Lane Rural, Multilane Rural Total, Injury, PDO CMF = EXP( 1 ) x [1+cos( )]^ 2 limits on 40 degrees two-lane 50 degrees - multilane Research Conclusions Critical Minimum Angle angle at which safety is substantially diminished and consideration be given to countermeasures AASHTO Green Book an angle of 60 degrees provides most of the benefits of a 90-degree intersection Recommendation revise to 75 degrees for two- lane roads and 90 degrees for multilane roads In line with some States Supported by physiology and visibility research Future Research Apply methodology to another State (transferability) Acquire data from urban area and apply methodology (no CMFs for urban intersections) Acquire data for more 3-leg intersections (no 3- leg CMFs) Feasibility of approach level analysis Alternative analysis methods to expand or support findings Questions? No Match Importance of Verification Right-angle misclassification (< 85 degrees) 24% of 4-leg 30% of 3-leg Skewed misclassification 3% of 4-leg 15% of 3-leg CMFs Derived Intersection & Collision Category NB Model Form Alternative Base Flexible Form 1 Flexible Form 2 4-Leg, All Sites, Total Crashes 4-Leg, All Sites, Injury Crashes 4-Leg, All Sites, PDO Crashes 4-Leg, Rural Sites, Total Crashes 4-Leg, Rural Sites, Injury Crashes 4-Leg, Rural Sites, PDO Crashes 4-Leg, Rural Two-lane Sites, Total Crashes 4-Leg, Rural Two-lane Sites, Injury Crashes 4-Leg, Rural Two-lane Sites, PDO Crashes 4-Leg, Rural Multilane Sites, Total Crashes 4-Leg, Rural Multilane Sites, Injury Crashes 4-Leg, Rural Multilane Sites, PDO Crashes 3-Leg, All Sites, Total Crashes 3-Leg, Rural Sites, Total Crashes 32 33