Study Details
Study Title: Evaluating the Effects of Freeway Design Elements on Safety
Authors: Park, Fitzpatrick, Lord
Publication Date:JAN, 2010
Abstract: Increased emphasis has been placed on improving the explicit role of highway safety in making decisions on highway planning, design, and operations. This can be achieved by quantifying the safety effects of geometric design elements for various highway facilities. The objective of this study was to investigate the safety effects of two important design elements for freeways: ramp density and horizontal curve. Data available for use in the evaluation included 324.2 centerline miles of freeways collected in Texas. Five years (1997-2001) of freeway crashes were examined. Negative binomial regression models were used to estimate the effects of independent variables on crashes. The final model for evaluation revealed that crashes on freeway segments were associated with ADT, on-ramp density, degree of curvature, median width with inside shoulder, the number of lanes (for urban freeways), and whether the freeway is in an urban or rural area. Off-ramp density was not statistically significant in the model. Furthermore, the effect of on-ramp density on freeway crashes was significant for horizontal curves but not for tangent sections indicating freeway designers should eliminate or minimize the number of on-ramps within the horizontal curves. The statistical modeling results were geared into the development of accidents modification factors (AMFs) for on-ramp density and horizontal curves that can be used for safety prediction of freeways.
Study Citation: Park, B.J., K. Fitzpatrick, and D. Lord. "Evaluating the Effects of Freeway Design Elements on Safety." TRB 89th Annual Meeting Compendium of Papers CD-ROM. Washington, D.C., 2010.
Study Report: Download the Study Report Document
CMFs Associated With This Study
Category: Access management
Countermeasure: Increase freeway on-ramp density from 0 to 1 ramps per mile
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| 1.05 | -5 |  | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | Urban |
| 1.047 | -4.7 |  | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | Urban |
| 1.033 | -3.3 | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | All |
Countermeasure: Increase freeway on-ramp density from 0 to 1 ramps per mile on curves
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| 1.04 | -4 | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | All |
Countermeasure: Increase freeway on-ramp density from 0 to 1 ramps per mile on tangents
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| 1.023 | -2.3 | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | All |
Countermeasure: Increase freeway on-ramp density from 0 to 10 ramps per mile
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| 1.636 | -63.6 | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | Urban |
| 1.578 | -57.8 | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | Urban |
| 1.379 | -37.9 | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | All |
Countermeasure: Increase freeway on-ramp density from 0 to 10 ramps per mile on curves
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| 1.48 | -48.1 | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | All |
Countermeasure: Increase freeway on-ramp density from 0 to 10 ramps per mile on tangents
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| 1.25 | -25.1 | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | All |
Countermeasure: Increase freeway on-ramp density from 0 to 5 ramps per mile
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| 1.279 | -27.9 | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | Urban |
| 1.256 | -25.6 | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | Urban |
| 1.174 | -17.4 | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | All |
Countermeasure: Increase freeway on-ramp density from 0 to 5 ramps per mile on curves
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| 1.217 | -21.7 | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | All |
Countermeasure: Increase freeway on-ramp density from 0 to 5 ramps per mile on tangents
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| 1.12 | -11.9 | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | All |
Countermeasure: Increase freeway on-ramp density from X to Y ramps per mile
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
 | | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | Urban |
| | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | Urban |
| | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | All |
Countermeasure: Increase freeway on-ramp density from X to Y ramps per mile on curves
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | All |
Countermeasure: Increase freeway on-ramp density from X to Y ramps per mile on tangents
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | All |
Category:Alignment
Countermeasure: Increase degree of curve on freeways from 0 to 10 (curvature in degrees)
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| 3.35 | -234.7 | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | Urban |
| 2.99 | -199 | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | All |
Countermeasure: Increase degree of curve on freeways from 0 to 15 (curvature in degrees)
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| 6.12 | -512 | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | Urban |
| 5.18 | -418 | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | All |
Countermeasure: Increase degree of curve on freeways from 0 to 5 (curvature in degrees)
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| 1.83 | -82.9 | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | Urban |
| 1.73 | -73 | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | All |
Countermeasure: Increase degree of curve on freeways from X to Y (curvature in degrees)
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|
| | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | Urban |
| | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways | All |
