Interstate 15 and Main Street, American Fork, UT 

DIVERGING DIAMOND INTERCHANGE 

THE PROBLEM 

The interchange at I -15 and 

Main Street experienced 

significant demand increases 

due to rapid population and 

commercial growth in the area. 

In addition, the conventional 

diamond interchange design 

had only a single lane and no 

left turn lane. It could take 

drivers 20-30 minutes to get 

through the interchange.

THE SOLUTION 

Installation of the Nation’s 

second Diverging Diamond 

Interchange.

THE OUTCOME 

⯀ The new DDI can comfortably 

accommodate 40,000 

vehicles per day, 10,000 

more than the conventional 

diamond interchange 

alternative. 

⯀ Illuminated pedestrian 

walkways are provided along 

both sides of Main Street 

through the interchange, 

and bicyclists can choose 

to ride in -lane or along the 

shoulders adjacent to the 

right lanes in both directions.

⯀ Overwhelmingly positive 

reaction from businesses and 

surrounding communities

due to reductions in 

congestion and delay.

CONTACT 

Jeffrey Shaw, P.E.

FHWA Office of Safety

202.738.7793

jeffrey.shaw@dot.gov

Mark Doctor, P.E.

FHWA Resource Center 

404.895.6210

mark.doctor@dot.gov

INTERCHANGE LOCATION 

40°22’36.3”N 111°49’07.6”W 

FHWA-SA-14-044 Revised July 2020

”

“

Right after we opened Pioneer Crossing, 

we did an extensive survey ... what is your 

perception of this DDI? ... the response 

was overwhelmingly ... 85-90 percent ... 

positive because they were comparing 

delay time ... and it was night and day. 

Background 

The Utah Department of Transportation 

(UDOT) selected the interchange located 

at Interstate 15 and Main Street in the city 

of American Fork for the Nation’s second 

Diverging Diamond Interchange (DDI) design 

to address rapid population and commercial growth in the area. The original interchange, a conventional diamond design, had 

become overwhelmed with increasing demand as the area grew. This led to significant corridor

congestion, with drivers on the cross street sometimes waiting up to 30 minutes to get through 

the interchange.1 

— Daniel Avila, P.E., Deputy Program Director 

Access Utah County, Utah DOT 

Challenges 

UDOT faced three challenges at this location. First, a new facility needed to support increasing demand on the corridor as the population continued to grow. Second, a new facility had to 

improve the flow of traffic along the corridor not only to facilitate the movement of goods and 

people, but also to address a growing concern about poor air quality—an issue that was exacerbated by lengthy queues of idling cars waiting their turn to proceed through the interchange. 

Finally, UDOT needed to be cost-conscious, implementing the most cost-effective solution to 

address mobility and environmental concerns. 

Approach 

UDOT investigated available options 

and focused on innovative designs 

because they offer a way to save money, 

improve capacity, and keep people 

moving so idling is minimized. UDOT’s 

initial simulation of the DDI indicated it 

would reduce backup by 50 percent in 

the first year and continue to function 

well as traffic grew over the next two 

View of the Pioneer Crossing DDI decades and beyond.2 

Source: DDI Case Study Video FHWA-SA-14-043 

Results 

Opening day traffic at the new DDI matched the highest demand projected in models and 

demonstrated that the design works – reducing previously crippling delay and accommodating an additional 10,000 vehicles a day.3

 Due to the lower number of phases, pedestrians and 

bicyclists also spend much less time waiting for the green signal than would be the case in a 

conventional diamond interchange design. Pedestrian sidewalks are provided along both sides 

of Main Street through the entire interchange. Bicyclists may ride in the traffic lanes or along 

shoulders adjacent to the right lanes in each direction. 

1 Interview with Eric Rasband, Salt Lake City, October 10, 2013. 

2 Utah DOT, UDOT Diverging Diamond Interchange (DDI) Observations & Experience, 

UT-12.05 (April 2012). Available at: https://digitallibrary.utah.gov/awweb/awarchive?item=55020

3 Ibid. www.safety.fhwa.dot.gov

This Fact Sheet is a companion to the Video Case Study (FHWA-SA-14 -043)