Diverging diamond interchanges (DDIs) have safety and operational benefits over conventional diamond interchanges. A DDI reduces conflict points from 26 to 14 by having traffic cross to the left side of the road at the interchange, before crossing back to the original side further down. Microsimulation modeling showed DDIs improved traffic flow with shorter signal cycles, less deceleration, and reduced vehicle queues. While innovative initially, over 1,000 DDIs have now been built worldwide to address increasing traffic demands safely and cost effectively.

1. Diverging Diamond Interchanges
Impact on Motorway Mainline Merges
Sam Gray, AITPM, August 2017

2. AGENDA
1. What are DDIs?
2. Innovation: History & Future
3. Conventional Interchanges: A Review
4. Onramp Motorway Arrival Flow & Safety
5. Microsimulation Results
Source: FHWASA
Need to Innovate
Urbanisation
Congestion
Safety
Funding

3. WHAT IS A DDI?
40% throughput increase
50% crash reduction
30+ year life
70% cost reduction

4. HISTORY OF INNOVATION
Safety
• 32 to 8 conflict points
• 0 crossing conflicts
• Lower speed environment
• 40% reduction in total collisions
• 90% reduction in fatal collisions
Operations
• Continuous flow
• 20-50% decrease in delay
Value for money
• No electrical hardware
• Maintenance
Precedence
• France (Half the worlds total)
• UK & Aus (1960s)
• USA (1990s)

5. FUTURE OF INNOVATION
Superstreet
CFI PFI
DDI

6. INNOVATION IN AUSTRALIA
QLD
• TMR endorsed
• DDI construction at Caloundra
• CFI construction at Gold Coast
NSW:
• Investigations of DDI & CFI
VIC:
• DDI in for Detailed Design,
• CFI approved for Hoddle St, VicRoads
endorsed
• MUARC CFI Research Study
ACT:
• Investigation of CFI
WA:
• ARRB Driver Simulation DDI Research
Project

7. INTERCHANGE EVOLUTION
Types
• Diamond  Single Point revolution
Safety
• Numerous conflicts points
• Fast speed environments
• Pedestrian distances on pavement
• Cyclists lack of segregation
• Ramp queue spillback
Operations
• Numerous conflicts  Multi-phasing Cycle times
• Bottlenecks  ramp queue spillback
• Pedestrian walk times
Upgrades
• Bridge widening costs / ramp widening costs
• Land acquisition costs
• Construction disruption to users
Diamond Interchange
Single Point Interchange

8. RECORDED SAFETY BENEFITS
CONVENTIONAL VS. DIVERGING DIAMOND
0
20
40
60
80
100
120
140
160
Front St / I-435, Missouri Dorsett Rd / I-270, Missouri
Crash/Year
Before Injury/Fatal
Before Property Damage
Before Total
After Injury/Fatal
After Property Damage
After Total

9. 260m merge
Layout
Demands and Patterns
20%
80%
30%
70%
3,300vph
1,050vph
LoS D
3,300vph
1,150vph
LoS E
3,300vph
1,200vph
LoS F
CONVENTIONAL VS. DIVERGING DIAMOND
MICROSIMULATION MODELLING SETUP

10. DDI CONFLICT REDUCTION
Source: Google
Conflict Type Crossing Merging Diverging Total
Conventional
Diamond
10 8 8 26
Diverging Diamond 2 6 6 14

11. DDI SIGNAL PHASING BENEFITS
Source: Google
# Phases Cycle Time
Model Conventional
Diamond
4 140s
Model Diverging
Diamond
2 80s
5%
15%
80%
Model DDI
2 Phase 80CT
6%
20%
74%
Model
Conventional
4 Phase 140CT

12. 61%
4%
35%
(49s)
54%
8%
39%
(31s)
62 intervals
where 8 or more
vehicles arrive
32 intervals
where 8 or more
vehicles arrive
48 intervals where less
than 2 vehicles arrive
27 intervals where less
than 2 vehicles arrive
CONVENTIONAL VS. DIVERGING DIAMOND
MICROSIMULATION MODELLING RESULTS

13. 61%
4%
35%
(49s)
54%
8%
39%
(31s)
92 intervals
where
acceleration
greater than
1m/s/s
44 intervals
where
acceleration
greater than
1m/s/s
0 interval where
deceleration is required
13 intervals where
deceleration is required
CONVENTIONAL VS. DIVERGING DIAMOND
MICROSIMULATION MODELLING RESULTS

14. Ramp Travel Results
Mainline Travel Results
CONVENTIONAL VS. DIVERGING DIAMOND
MICROSIMULATION MODELLING RESULTS

15. CONVENTIONAL VS. DIVERGING DIAMOND
MICROSIMULATION MODELLING RESULTS
Network Results

16. DDIS WORLDWIDE
Open
87+
Construction
15+
Design
20+
Planning
30+
0
20
40
60
80
100
120
140
Jun-09
Dec-09
Jun-10
Dec-10
Jun-11
Dec-11
Jun-12
Dec-12
Jun-13
Dec-13
Jun-14
Dec-14
Jun-15
Dec-15
Jun-16
Dec-16
Jun-17
Dec-17
Jun-18
Cumulative DDI Openings
Recorded Forecast

17. REFERENCES
1. Diverging Diamond Interchange Informational Guide, FHWASA, 2014
2. AASHTO_DCD –factsheet, Additional Selected Technologies, Double Crossover Diamond (DCD) Interchange, 2013
3. Diverging Diamond Interchange Performance Evaluation, Missouri Department of Transportation, 2011
4. Safety Evaluation of Diverging Diamond Interchanges in Missouri, MDOT, 2015
5. DDI Guideline, A UDOT Guide to Diverging Diamond Interchanges, 2014
6. Design and Operational Performance of Double Crossover Intersection and Diverging Diamond Interchange,
Transportation Research Board (TRB)
7. High-Resolution Controller Data Performance Measures for Optimizing Diverging Diamond Interchanges and
Outcome Assessment with Drone Video, Transportation Research Board (TRB), 2014
8. Modeling Diverging Diamond Interchange Under Constraints of Ramp Metering, HOV lane and BRT Transit, Federal
Highway Administration, 2015
9. Advanced Transportation Solutions, a division of American Consulting (ATS/American)
http://www.divergingdiamond.com/index.html
10. KwaMashu Interchange Upgrade, Civil Engineering Magazine, December 2014,
http://www.civildesigner.com/press/kwamashu.pdf
11. Highway Capacity Manual (HCM) Methodology for Alternative Intersections / Interchanges, Federal Highway
Administration (FHWA), 2013
12. Design and Operational Performance of Double Crossover Intersection and Diverging Diamond Interchange,
Transportation Research Record: Journal of the Transportation Research Board, No. 1912, 2005
13. UDOT Diverging Diamond Interchange (DDI) Observations & Experience, UDOT, 2012