1. DDI : The Ops Experience
A Region 3 Perspective

2. DDIs in Region 3
• American Fork
– Pioneer Crossing and 500 E
– Purpose built
– Work great
– Run free

3. DDIs in Region 3
• Timpanogos Highway (SR-92)
– Retrofit DDI
– Under the freeway
– Was opened with less than designed lanes
– Works great (most of the time)
– We’re retrofitting a trail into the DDI

4. DDI Operational Modes
• Excess Capacity Mode (Low V/C ratio)
– Run the intersections uncoordinated
– Snappy response on detection
– Low investment in timing – spend efforts on
best possible detection
– Low delay – easy to operate
– American Fork DDIs

5. DDI Operational Modes
• Design volume mode (moderate V/C ratio)
– Run the intersections coordinated
– Coordination is relatively easy
– Low delay – fun to operate
– Timpanogos Highway

6. DDI Operational Modes
• Near Capacity Mode (high V/C ratio)
– Coordination is crucial, difficult and time
consuming
– Interchange may “lock up”
– Small changes = big delays
– Large through movements are a problem

7. Observations
• DDI vs. SPUI
– Power out/Flash operations DDI is better
– Off-peak operations DDI is better
– Overcapacity ops – SPUI is better
– Frontage road spacing – SPUI is better
– Pedestrians – DDI works great with center
sidewalk

8. Observations
• With both intersections
on same cabinet:
– Diagram in cabinet
shows indications
– Camera Access in
cabinet
Indicators show
which lights are
on

9. Pedestrians
• Timpanogos Highway
– Adding trail to south side of interchange
– Not impossible – but would be better in center
– Adding peds on side rather than center can
drive a decision to add a second controller
and cabinet.

10. Acknowledgements
• Eric Rasband and the I-15 CORE team
• Matt Luker and the UDOT Traffic Management Division
• Further Reading:
– UDOT Diverging Diamond Interchange (DDI) Observations &
Experience, Eric Rasband, Taylor Forbush and Kelly Ash