Kyeil Kim, Ph.D., PTP

Atlanta Model Users Group

August 25, 2017

Travel Impact of

I-85 Bridge Collapse

through

Activity-Based Modeling

Location

Aftermath

• The bridge

collapsed in

the evening of

March 30

• April 3,

Monday PM

Status on April 10

Source: www.wsbtv.comSource: www.511ga.org

Reconstruction

Source: https://app.oxblue.com/open/gadot/i85bridgerebuild

• The bridge

reopened on

May 12

The Study

• Purpose

– Obtain a general idea of what the impact of the I-85 bridge collapse on the metro Atlanta traffic is, from travel demand modeling

• Network Assumptions

– Network condition: Evening of March 30

– I-85 NB/SB links at Piedmont Road taken out

– Buford Spring Connector at Piedmont Road closed

– Piedmont Road closed at I-85

Area Traffic Profile (AADTs, 2016)

308,000Station 067-2738

136,000Station 121-5450

190,000Station 121-6370

404,000Station 121-5482

297,000Station 121-5468

209,000Station 121-5452

302,000Station 135-6285

223,000Station 089-3332

200,000Station 089-3361

229,000Station 121-5970

177,000Station 089-3292

Source: http://geocounts.com/gdot/

Methodology

• Project-Based Approach

– Not using feedback looping (Not the LRP-style model runs)

– Steps

1. Run 1 feedback, no-build scenario (i.e., normal condition with

bridge) with 100% sample

2. Fix O/D patterns

3. Update highway/transit skims from the “bridge collapse” scenario

4. Run mode choice

5. Highway/transit assignments

Project-Based Approach

Source: Jonathan Nicholson, 2016

No Build Scenario

Full Model Run

Travel Skims

Long Term Models

Mode Choice, Stop

Location, …

Build Scenario

Partial Run

Travel Skims

Mode Choice, Stop

Location, …

TransitTransit

Daily Patterns,

Tour Frequency, …

HighwayHighway

Daily Patterns,

Tour Frequency, …

SOV

HOV

Transit

SOV

HOV

Transit

Assignment

Assignment

Number of choices

held in memory

AM Peak Period – Highway Assignment (2017)Normal Bridge Gone

AM Peak Period – Highway Assignment (2017)

Difference in traffic

volumes between “bridge

collapse” and “normal”

conditions, color-coded

with congested speeds

AM Peak Period –

Highway Assignment

(2017)

% Change in AM traffic

volumes

-7%

-22%

-28%

-3%

-28%

-2%

-0.3%

2%

16%6%

12%

2%

Select Link Analysis

Origins/Destinations of

traffic using selected link

NormalCondition

(AM peak period)

Selected Link(southbound)

SLA on I-85 SB (AM Peak)Normal Bridge Gone

SLA on I-85 SB (AM Peak)Normal Bridge Gone

SLA on GA400 SB (AM Peak)Normal Bridge Gone

SLA on GA400 SB (AM Peak)Normal Bridge Gone

SLA on I-75 SB (AM Peak)Normal Bridge Gone

SLA on I-75 SB (AM Peak)Normal Bridge Gone

SLA on Downtown Connector SB (AM Peak)Normal Bridge Gone

SLA on I-75 NB (AM Peak)Normal Bridge Gone

Systemwide Impact

• The model indicates …

– Total person trips

– Total vehicle trips

– Total transit trips

– Total daily VMT

– Total daily VHT

– Daily congested average speed

– Average delay per person

Systemwide Impact (2017 Model Estimates)

Total trips, VMT, VHT (20-county area)

Trips by mode (20-county area) VMT by vehicle class (20-county area)

Measure With Bridge Bridge Collapse Difference % Difference

Total person trips 19,922,000 19,911,000 -11,000 -0.1%

Total vehicle trips 16,425,000 16,403,000 -22,000 -0.1%

Total transit trips 343,000 356,000 13,000 3.8%

Total VMT 163,988,000 163,701,000 -287,000 -0.2%

Total VHT 5,357,000 5,419,000 62,000 1.2%

With Bridge Bridge Collapse

Mode Trips Trips Difference % Difference

Drive alone 10,215,000 10,198,000 -17,000 -0.2%

Shared ride 3,046,000 3,042,000 -4,000 -0.1%

Walk & Bike 993,000 995,000 2,000 0.2%

Walk Transit 222,000 228,000 6,000 2.7%

Drive Transit 121,000 129,000 8,000 6.6%

School Bus 956,000 956,000 0 0.0%

Total 15,554,000 15,547,000 -7,000 0.0%

With Bridge Bridge Collapse

Vehicle Class VMT VMT Difference % Difference

Single occupant cars 105,032,000 104,789,000 -243,000 -0.2%

Carpools 20,394,000 20,239,000 -155,000 -0.8%

Trucks 37,693,000 37,741,000 48,000 0.1%

Drive access to transit 869,000 931,000 62,000 7.1%

Total 163,988,000 163,701,000 -287,000 -0.2%

Acknowledgement

• Raghuprasad Sidharthan, WSP

• Jonathan Nicholson, Atkins

• Rosella Picado, WSP

• Abigail Marinelli, ARC