DDI : The Ops ExperienceA Region 3 Perspective

DDIs in Region 3• American Fork

– Pioneer Crossing and 500 E– Purpose built – Work great– Run free

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

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

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

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

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

Observations• With both intersections

on same cabinet: – Diagram in cabinet

shows indications– Camera Access in

cabinet

Indicators show which lights are

on

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.

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