Project 0-6997: Develop Capacity and Cost Benefits of Super 2 Corridors
Marcus Brewer, Project Supervisor
94th Annual Transportation Short Course – October 13, 2020
Acknowledgements
• TTI Research Team• Marcus Brewer• Tim Barrette• Kay Fitzpatrick• David Florence• Brianne Glover• Steven Venglar
• TxDOT PMC• Wade Odell (RTI)• Matt Evans (FTW)• Andrew Holick (BRY)• Cathy Kratz (AUS)• Ana Mijares (LFK)• Rebecca Wells (ATL)• Mark Wooldridge (HOU)
Project Objectives (2-year project)1. Evaluate additional capacity and reductions in % time
following for Super 22. Determine capacity and % time following of fixed-length
Super 2 corridors, varying number, length, and spacing of passing lanes
3. Compare Super 2 to traditional 4U, 4D, and 2U+LT4. Compare incremental economic benefits of Super 2 vs
widening to four lanes.
Task 2• Super 2 / 2+1 highways are used around the
world for a range of traffic volumes• High truck volumes can still cause issues,
specifically at merge points• Effective passing lane length is significantly
longer than physical passing lane length• Operational analysis frequently done by
microsimulation • Previous research had not put a monetary
value on the use of Super 2 configurations
Identify Relevant Influences on Performance• Capacity and
percent time following
• Economic benefits
• Analysis methods and tools
Task 3
Identify Study SitesCollect Field DataProcess Field Data
Districts with Super 2s:
AustinBryan
ChildressCorpus Christi
El PasoLaredoParisPharr
San AntonioWichita Falls
Yoakum
• Site selection focused on sites with:
• Lower volume = may have recently been expanded from two lanes
• High volume = serious consideration for expansion to four lanes
• Truck volume = influences on operations
• Five sites • South and West Texas• 70-75 mph
• Field data predominantly based on tubes and digital video, plus field notes
Identify Study SitesCollect Field DataProcess Field Data
Task 3
• Road tube volumes similar to TPP ADT counts at 3 of 5 sites
• Percent of following vehicles higher in passing lane, particularly at upstream end
• Speeds in passing lane higher than right lane, but generally by ≤ 3 mph
• At 4 of 5 sites, speeds in both lanes higher than upstream or downstream 2-lane section
Identify Study SitesCollect Field DataProcess Field Data
Task 3
Preliminary Findings
Task 4 Simulation
• Primary purpose = analyze operational characteristics to identify relative benefits of:
• Super 2 corridors (2S)• Four-lane cross-sections (4U, 4D)• Two-lane cross-sections with auxiliary turning
lanes (2U+LT)
• VISSIM 2020• Latest lane-change distribution updates model
merging behavior at the end of passing lanes• Allowed passing on opposite direction for some
cross-sections• 24-hour simulation to provide daily data for
economic analysis in Task 5
Proposed measures:• Delay• Speeds• % time following
Simulation Corridor
• 40-mile generic rural highway corridor• 9 ADT levels (3K-19K in 2K increments)• 3 truck percentages (20, 30, 40)• 8 cross-sections
Code Cross‐Section Passing Lane Length
Number of Passing Lanes
2U 2‐lane undivided None None2U+LT 2‐lane undivided with
left‐turn lanes at highway intersections
None None
4U 4‐lane undivided None None4D 4‐lane divided None None2S‐23 Super 2 2 miles 3 passing lanes2S‐33 Super 2 3 miles 3 passing lanes2S‐26 Super 2 2 miles 6 passing lanes2S‐36 Super 2 3 miles 6 passing lanes
Length & Cross-Sections
Minimum Hourly Average Speed Results
2S-36 had best Super 2 performance2S-26 outperformed 2S-33 in every scenario
Total Network Delay Results
Performance of 2U+LT similar to 2U for ADTs < 15000, but it stabilized at higher volumes
Percent Time Spent Following
PTSF does not vary widely, particularly at lower volumes and truck percentages
Additional Findings
• 4D similar to 4U at ADT ≤ 13,000, but higher volumes susceptible to effects of trucks and turning vehicles
• Observed trends support safety research: alternatives that include Super 2 or turning lanes are often preferable to 4U
• Relative performance of Super 2 alternatives supports previous research: adding more passing lanes provides more benefit than adding length to passing lanes
Providing turning lanes may be more beneficial in operations, as well as safety, than providing passing lanes, if a choice must be made between the two
Task 5
• Baseline Scenario vs Project Scenario• Scenario choices:
• Undivided 2-lane • Super 2• Undivided 4-lane • Divided 4-lane
• Cost = project construction cost• Average cost per lane-mile calculated using historic
project cost examples for each scenario• Benefit = based on VHT savings and safety
• Value of time ($/hour)• Vehicle operating cost savings ($/hour)• Environmental benefits ($/hour)• Safety benefit (crash reduction)
• Model analysis based on Task 4 outputs
Conduct Economic Analyses
Task 5• Spreadsheet model
• Compares base (no-build) vs. project (build) scenario
• Default values• 20-year operating period• 40-mile corridor length• 2-year construction period• 3% discount rate• 2018 CY dollars
• Produces cost/mile for each benefit and cost• Calculates benefit/cost ratio (BCR) and net
present value (NPV)
Key Model Inputs:• Project Type
(cross-section)• Project Length• AADT• % Trucks
Task 5
Model Inputs:• Yellow cells =
user input• Gray cells =
default values that can be overridden
Task 5
Model Outputs:• Benefits over 20-
year project life• Discounted
project cost• BCR and NPV
Task 5
BCR Results:• Net benefits for
all at high ADT• Minimal benefit
for 4-lane at low ADT
• 2S-36 best results
Project Type
3,000 ADT 19,000 ADT
20% Trucks 40% Trucks 20% Trucks 40% Trucks 2S‐23 2.1 2.2 26.2 70.62S‐33 2.2 2.3 28.6 73.82S‐26 2.3 2.5 33.9 80.62S‐36 2.4 2.5 40.1 87.74U 0.2 0.2 6.2 13.44D 1.0 1.0 5.9 26.2
Task 5
NPV Results:• Similar trends to
BCR, though not identical
• Better NPV for 4-lane at high ADT, high truck %
Project Type
3,000 ADT 19,000 ADT
20% Trucks 40% Trucks 20% Trucks 40% Trucks 2S‐23 $42 $46 $977 $2,700
2S‐33 $45 $49 $1,072 $2,825
2S‐26 $52 $56 $1,277 $3,090
2S‐36 $55 $59 $1,517 $2,264
4U ($191) ($185) $1,271 $3,062
4D ($7) ($1) $829 $4,236 Note: values shown in millions of 2018 dollars
Summary of Findings
• Super 2s compared favorably (operationally and economically) to all alternatives; 2S-36 the best Super 2 performer
• Turning lanes provide favorable operational benefit at higher through volumes
• 4-lane alternatives provide higher speeds and lower delay at lower volumes, but are not economically favorable at those volumes
• 4-lane can offer high NPV at high volumes and truck percentages, but not necessarily BCR, compared to Super 2
Design process should consider the presence of turning vehicles, not just through vehicles, when evaluating cross-section alternatives.
Guidelines (0-6997-P1)
• Summarizes the findings and guidance from 20 years of TxDOT Super 2 research (0-4064, 0-6135, 0-6997) and related external studies
• Includes key material from RDM and MUTCD, plus references to those documents for more details
• 4 chapters• Introduction• Selection of Alternatives• Design Considerations• Traffic Control Devices
• Intended as primary source for practitioners
Guidelines (0-6997-P1)
• Background discussion• Typical environment for Super 2s• Broad description of potential benefits
• What is a Super 2?• RDM definition: one in which a periodic passing
lane is added to a two-lane rural highway to allow passing of slower vehicles and the dispersal of traffic platoons
• Potential configurations (more flexible than 2+1)• Distinction from climbing lanes
IntroductionSelection of AlternativesDesign ConsiderationsTraffic Control Devices
Guidelines (0-6997-P1)
• Operational measures• Volume/speed/delay/PTSF• Truck %• Terrain• Through vehicles vs. turning vehicles
• Safety measures• CMF (sourced from Clearinghouse and other
studies)• Economic measures
• Benefits (operating and time cost savings, safety, environmental)
• Construction costs• Tied to 0-6997 economic analysis and
spreadsheet tool
IntroductionSelection of AlternativesDesign ConsiderationsTraffic Control Devices
Guidelines (0-6997-P1)
• Basic principles from RDM• Considerations for the designer• Design criteria• Taper lengths • Separation distance
IntroductionSelection of AlternativesDesign ConsiderationsTraffic Control Devices
Guidelines (0-6997-P1)
• Cross-section configurations
• Passing lane length• Passing lane spacingIntroduction
Selection of AlternativesDesign ConsiderationsTraffic Control Devices
Guidelines (0-6997-P1)
• Source documents:• TMUTCD• Traffic Standard Sheets TS2-1-18
and TS2-2-18 • Signing
• In passing lane• Advance signing
• Markings• Centerline• Taper transitions• Buffer between lanes in opposite
direction
IntroductionSelection of AlternativesDesign ConsiderationsTraffic Control Devices
Next Steps• Future research
• Further development of the BCA tool with additional project data
• Refine CMF for Super 2 (problem statement developed for consideration)
• Analysis of the 4D+LT cross-section• Proposed implementation
• Virtual workshops on BCA spreadsheet tool• Virtual workshops on Guidelines document