SAFETY ANALYSIS OF DRIVEWAY CHARACTERISTICS IN SOUTH CAROLINA USING GIS Wayne A. Sarasua Kweku Brown
Overall Goal The overall goal of this project is to enhance SCDOT s current access management practices resulting in a reduction in crashes, injuries, and fatalities on South Carolina roadways as well as improved traffic flow
Literature Review Access Management studies generally fall into three categories : Impacts on Safety Median Access point density Sight distance Intersection & Interchange Turn prohibition Impacts on Traffic Operations Level of service Capacity Impacts on Residents and Business
Literature Review Access point density: Schultz et al., Safety impacts of access management techniques in Utah, (2007) evaluated the safety performance of arterials in which access management techniques have been implemented. They concluded that the relationship between access points per mile and crash rate has a positive correlation in Utah. Frawley and Eisele, Crash analyses of raised medians and driveway density: How access management makes communities safer, (2004) completed an evaluation of 11 case studies in Texas and Oklahoma and showed that implementing raised medians and performing driveway consolidation would reduce the potential number of conflicts points along roadways. Conclusion: reducing access point density reduces crash rates, reduces speed differential between vehicles, enhances operation and could improve roadway capacity and reduce need of new capacity improvement
Literature Review Medians: Gattis et al., Roadway median treatments, (2010) examined relationships among crash rates and different types of median (None with occasional left turn lanes, two-way left-turn lane, Raised, Depressed) of roadways in Arkansas. They concluded that raised or depressed median yield the lowest crash rates. Mauga and Kaseko (2010) evaluated and quantified the impact of types of medians, including raised medians and two-way-left-turn-lanes on traffic safety in the midblock sections. The results showed that segments with a raised median had lowered the crash rate by 23% compared to segments with a TWLTL. Schultz et al., Safety benefits of median installations in Utah, (2012) analyzed safety at locations where raised medians are installed. They concluded that raised medians significantly reduce crash frequency and severity. Frawley, Raised median economic impact and safety research findings: Application for Cooper street, (2010) studied the benefit of converting a two-way left turn lane to a raised median for particular locations in Bryan, Temple and Tyler, Texas. The benefits include lower crash frequency, fewer crashes involving left-turns, no migration of crashes to intersections, very few U-turn crashes, and fewer severe crashes. Conclusion: Raised medians reduce crash frequency and severity with no migration of crashes to intersections. Traffic operations are improved.
A Methodology for Determining the Economic Impacts of Raised Medians William L. Eisele, P.E., and William E. Frawley, AICP (TTI 1999) Business owners who were present before, during, and after the median installation felt that their regular customers continued to use their businesses In contrast, those businesses that were interviewed prior to the installation of the raised median thought their customers would be less likely to continue to use their businesses Perceptions appear worse than reality
Data Collection Procedures and Data Summary
30 Preliminary Corridors
SC Geocoded Crashes
Rear-end and angle crashes on US 25 in Greenville, SC for 2010 (left) and 2012 (right)
Symbol Table 2010 2011 2012 Driveway Related Crash Angle Crash Rear-end Crash
Greenville US 25 Length - 66 Driveway Crashes - 309 Rank 2 Rear End - 880 Angle 693 Clusters 9
Greenville US 25
Greenville SC 146 Length 13.9 Driveway Crashes - 294 Rank 3 Rear End 676 Angle 373 Clusters 20
Greenville SC 146
Greenville SC 146
Greenwood US 25
Richland US 1 Length 22.0 Driveway Crashes - 353 Rank 1 Rear End 1270 Angle - 715 Clusters - 30
Richland US 1
Before and After Results
Richland US 176
Final 11 Corridors COUNTY ROUTE TYPE ROUTE NUMBER LENGTH (MILES) 3 YEAR DRIVEWAY AVG RANK DRIVEWAY CRASHES Richland US 1 18.5 1 353 Greenville US 25 18.7 2 309 Greenville SC 146 13.5 3 294 Richland US 176 15.8 4 274 Lexington US 1 17.6 5 214 Horry US 17 55.4 6 195 Spartanburg SC 9 15.8 7 173 Greenville US 29 15.4 8 159 York US 21 35.6 9 147 Berkeley US 17 18.8 11 149 Florence US 52 20.4 12 131
Map of 11 Corridors
Corridor Analysis Data Collection Process Geometric Data were collected for selected corridors in three categories Segment -intersection to intersection Driveway Intersection All data were geocoded into ArcGIS
Sample Driveway with Attributes
Driveway Spacing
Skew Angles
Safety Analysis
Right In Right Out Buffers
Full Access Buffers
Proximity Analysis Average Distance from Reported Route by Year Average Distance (FT) Route 2010 2011 2012 US 1, Richland 14.6 3.7 3.2 US 25, Greenville 17.8 2.4 1.3 SC 146, Greenville 18.6 1.8 1.0 US 176, Richland 15.3 1.7 1.1 US 1, Lexington 14.7 4.4 4.7
Buffers with Crash Overlay
Annual Crash Frequency by Driveway Class
Annual Crash Frequency Full Access vs Right-in, Right-out
Negative Binomial Estimation Results for Crashes per Driveway
Negative Binomial Estimation Results for Crashes per Driveway Variables Estimate Std. Error z-value p-value (Intercept) -19.56 1.220e+00-16.038 < 2e-16 D_Spacing -0.0004154 2.809e-04-1.479 0.139233 D_Width 0.02656 2.448e-03 10.851 < 2e-16 N_Entry_Ln -0.3245 1.789e-01-1.814 0.069658 RaisedMedian -0.7094 3.238e-01-2.191 0.028457 D_Class4 0.7759 9.253e-02 8.386 < 2e-16 D_Class5 0.8610 1.507e-01 5.713 1.11e-08 D_Control 1.381 1.813e-01 7.622 2.51e-14 Ln(AADT) 1.668 1.058e-01 15.771 < 2e-16 SpeedLimit 0.01300 9.735e-03 1.335 0.181835 FAorRIRO 0.8114 2.329e-01 3.484 0.000494
Crash Modification Factors Variables CMF 95% Confidence Bounds Median (1 for raised, 0 for all others) 0.49 0 1.13 D_Class4 (High Turnover) 2.17 1.99 2.30 D_Class5 (High Turnover) 2.37 2.07 2.60 D_Control (Signalized) 3.98 3.62 4.44 FAorRIRO (Full Access or Right-in-right-out) 2.25 1.79 2.71
Crash Modification Functions
Crash Modification Functions Driveway Width 4 3.5 3 CMF DW 2.5 2 1.5 1 0.5 0-60 -40-20 0 20 40 60 DW a -DW b
Crash Modification Function
Crash Modification Function
Safety Performance Function
Crash Modifications Function
Corner Crash Analysis
Corner Clearance Annual crash frequency 0 to 150 feet from an intersection vs crash frequency 150 feet to 300 feet Distance from Intersection US 1 Richland US 25 Greenville SC 146 Greenville US 176 Richland SC 9 Spartanburg US 17 Berkeley # of driveways HP 2012 Crashes Crash frequency 0-150ft 150-300ft 0-150ft 150-300ft 0-150ft 150-300ft 238 124 112 32 0.47 0.26 188 141 169 45 0.90 0.32 53 42 75 38 1.42 0.90 117 95 74 63 0.63 0.66 100 74 58 22 0.58 0.30 113 86 37 5 0.33 0.06
Other Project Components Waiver Analysis Operational Analysis Economic and Benefit Cost Analysis
Recommendations for SCDOT ARMS
Sample Specific Changes to ARMS
Findings & Conclusions
Problems result when developers circumvent standard adherence through the waiver process Comprehensive driveway database critical to analysis Precise crash data is critical to analysis Makes it possible to associate crashes with driveways Can also facilitate corridor level and even statewide analysis with more robust results Reading crash narratives is time consuming and it is sometimes difficult to associate a crash with a particular driveway Use junction type with caution. Many driveway related crashes may be overlooked.
Research Golden Nuggets for Practical Application Research Findings: Raised medians can cut driveway related crashes in half (CMF of 0.49). Right-in, right-out driveways eliminate 3 conflict points that are the most critical with respect to crash severity and RIRO driveways have less than half the crash frequency of full access driveways. Reducing access point density improves roadway capacity and reduces need for new capacity improvements. Driveways within 150 of a an intersection have nearly twice the crash frequency of driveways 150 to 300 from an intersection.
Research Golden Nuggets for Practical Application (cont.) Better access policies result in lower crash severity, not just frequency. Approval of access waivers from adherence to good design practice often result in higher crash frequency. High turnover land use such as fast food restaurants have much higher crash frequency over other land uses such as small businesses.
Thank you! Questions?