TxDOTX-Ramps vs Diamonds: Safety and Operational Issues for Conversion

October 10, 2017

Presented by: Karen Dixon, Ph.D., P.E.

Outline

• Objective of this Safety Investigation

• Phase I Update

– Site Selection

– Data Collection & Analysis

– Findings to Date

• Current Phase II Activities

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Objective of Safety Investigation

Evaluate the safety implications of ramp configurations

and adjacent driveway placement for frontage roads in

Texas. Ramp configurations include traditional as well

as X-configuration ramps. Pay particular attention to

the position/influence of driveways.

Note: Phase I focus is on frontage road safety performance.

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Traditional (Diamond) Ramps

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Interchange

#1

Interchange

#2

Highway / Freeway

Weave / Merge on Freeway

X-Ramp Configurations

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Interchange

#1

Interchange

#2

Highway / Freeway

Weave / Merge on Frontage Road

Site Selection / Identification

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Site Selection -- Developed GIS Intersection Points for Freeways and Arterials

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Site Selection (continued) – Refined Interchange Identification using GIS Tools

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a) Aerial View b) GIS Street Map View

c) Applying 60 m Buffer d) Consolidated Intersection Point

Site Selection (continued) -- Developed a Stratified Random Sample

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Selection of Study Sites• Acquired full detailed data for 93 randomly selected

locations (62 sites with X ramps and 31 sites with traditional)

• Acquired additional general data for approximately 30 traditional diamond sites (to be included in Phase II)

• Identified urban frontage road driveway and ramp configurations upstream and downstream of each randomly selected interchange

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Data Collection and Documentation

• Confirmed Study Site Matches Study Objectives

• Identified Site Boundaries

• Acquired Data from RHiNo and GoogleEarth/Streetview

• Assembled Companion Crash Data for Each Study Location

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Additional Site Data – Aerial Photographs / Street View

1. Used GoogleEarth to “tag” key locations of interest

2. Developed Excel macro to measure distances between “tag” locations

3. Determined driveway density, proximity, and opportunity for influencing weaving at each study location

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Example Site Boundaries

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Identify Key Drivewaysand Gore Locations

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Identify Locations

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CL Reference Point 2B (out of view to left) – 0 ft

DW B4 - Driveway #4 (Center) – 874.9 ft

ON B1 – Painted Gore for On-Ramp B1 – 1160.9 ft

NG B1 – Physical Nose of Gore B1 – 1257.9 ft

BG B1 – Back of Gore for B1 – 1299.2 ft

DW B5 – Driveway #5 (beyond potential weave section) – 1556.2 ft

Driveways that Influence Weave –Traditional Ramps

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Interchange

#1

Interchange

#2

Highway / Freeway

Driveways in

Weaving Area

Driveways in

Weaving Area

Region Not

Directly Affected

Driveways that Influence Weave –X-Ramps

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Interchange

#1

Interchange

#2

Highway / Freeway

Driveways in

Weaving Area

Driveways not

Directly in

Weaving Area

but may still have

residual impacts

Region Not

Directly Affected

Summary of Data Elements Acquired

• Ramp Type

• Segment Length

• Distance between Gore Noses

• Gore Ratio (i.e. gore distance / segment length)

• Total segment driveways (and locations)

• Gore driveways (and locations)

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Summary of Data Elements Acquired (continued)

• Access density along road (points per mile)

• Access density within gore regions (points per mile)

• Distance from gore nose to first upstream and downstream driveways for G1 and G2

• Number of lanes

• ADT

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Example Crash Data (Site 158)• CRIS maps the crash to freeway centerline

• Direction of travel and crash type used for additional frontage road assignments (current activity)

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Conduct a Safety Assessment• Assemble Merged Database Suitable for Descriptive

Statistics for 3 years, 5 years, and 7 years (last year is 2015)

• Use the Merged Database to Conduct Statistical Safety Analyses– Assessed dates associated with each location

– Document and contrast X-ramps and traditional ramps at urban diamond interchange locations

– Determine overall safety impacts for total crashes as well as F+I crashes

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Review of Findings: X-Ramp F+I Crashes (3 yr)

• Critical variables include:

– Gore Ratio (i.e. length of gore divided by length of segment)

– Number of Driveways in Gore Region (within 400’ or gore physical nose)

– Upstream distance from Nose of G1 to closest driveway

– Prevailing Number of lanes between G1 and G2

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Review of Findings: X-Ramp Total Crashes (3 yr)

• Critical variables include:

– Gore Ratio

– Number of Driveways in Gore Region (within 400’ or gore physical nose)

– Upstream distance from Nose of G1 to closest driveway

– Downstream distance from nose of G2 to closes downstream driveway

– Prevailing Number of lanes between G1 and G2

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F+I Equation for X-Ramps

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Parameter Estimates

Parameter Estimate Standard Error t Value Approx

Pr > |t|

Intercept 0.046317 0.358278 0.13 0.8971

GoreRatio 2.089092 0.464500 4.50 <.0001

GoreDWs 0.039265 0.011883 3.30 0.0010

DupG1 -0.000323 0.000153 -2.11 0.0352

LnsG1toG2 0.343152 0.086948 3.95 <.0001

Total Crash Equation for X-Ramps

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Parameter Estimates

Parameter Estimate Standard Error t Value Approx

Pr > |t|

Intercept 1.041589 0.219128 4.75 <.0001

GoreRatio 2.580698 0.298749 8.64 <.0001

GoreDWs 0.023180 0.008454 2.74 0.0061

DupG1 -0.000426 0.000096911 -4.40 <.0001

DdownG2 -0.000454 0.000115 -3.94 <.0001

LnsG1toG2 0.394799 0.051814 7.62 <.0001

Review of Findings: Traditional (Diamond) Ramp F+I & Total

Crashes (3 yr)• Critical variables include:

– Number of Driveways in Gore Region

– Prevailing Number of lanes between G1 and G2

Note: Crashes within 250 of intersection excluded as they were considered crashes due to the intersection (per HSM recommended procedure)

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3 Year F+I for Traditional Ramps

Parameter Estimates

Parameter Estimate Standard Error t Value Approx

Pr > |t|

Intercept -1.535518 0.555519 -2.76 0.0057

GoreDWs 0.066647 0.018193 3.66 0.0002

LnsG1toG2 0.878466 0.204930 4.29 <.0001

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3 Yr Total for Traditional Ramps

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Parameter Estimates

Parameter Estimate Standard Error t Value Approx

Pr > |t|

Intercept -0.920540 0.289482 -3.18 0.0015

GoreDWs 0.065629 0.009286 7.07 <.0001

LnsG1toG2 1.175524 0.104178 11.28 <.0001

Example Calculation• X-Ramp Configuration with:

– Number of Driveways in Gore Region = 5

– Gore Ratio = 0.3 (so 30% of segment is in gore region –defined as 400 ft upstream and downstream of gore nose)

– Distance to closest upstream driveway from G1 = 385 ft

– Distance to closest downstream driveway from G2 = 360 ft [only critical for total crashes

– Number of lanes between gores (at weave location) = 3

• Predicted number of F+I crashes is 6 (so 2 per year)

• Predicted number of Total crashes is 24 (so 8 per year)

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Example Calculation• Traditional Ramp Configuration with:

– Number of Driveways in Gore Region = 5

– Gore Ratio = 0.3 (so 30% of segment is in gore region)

– Distance to closest upstream driveway from G1 = 385 ft

– Distance to closest downstream driveway from G2 = 360 ft [only critical for total crashes

– Number of lanes between gores (at weave location) = 3

• Predicted number of F+I crashes is 4 (so just over 1.3 per year)

• Predicted number of Total crashes is 19 (so just over 6.3 per year)

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Comparison of Example Values for Frontage Road Segment Crashes

F+I predicted crashes per year

(for segment)

Total predictedcrashes per year

(for segment)

X-Ramps 2 8

Traditional Ramps 1.3 (round to 2) 6.3 (round to 7)

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Additional Observations• Number of driveways in gore region critical for both

X-ramps and Diamond ramps

• X-ramp configurations, given the same characteristics, can be expected to result in slightly more frontage road crashes than for diamond applications

• Number of frontage road lanes is always influential when predicting frontage road segment crashes

• Phase II (currently underway) -- Evaluating the safety effects of the adjacent freeway segments

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Phase II Activities

• Assess the safety effects of the adjacent freeway segments (both at X-ramp and Diamond ramp locations)

• Develop self-calculating spreadsheet for equations

• Assemble charts to demonstrate site characteristic sensitivity

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Questions?

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Karen Dixon, Ph.D., P.E.

Phone: 979-845-9906

Email: k-dixon@tti.tamu.edu