Angela from Aurora writes, “What’s driving you crazy? I have a traffic intersection that is driving me crazy, and I was thinking you might find it interesting. Let's say you’re exiting northbound I-225 onto Alameda and you want to make a U-turn to get back onto the highway in the southbound direction. But when you go and make your U-turn, you end up turning onto the ramp while people making the right turn from Alameda have a green arrow at the same time as my green left turn arrow. There are no signs indicating the U-turn is not allowed. I feel like this is just an accident waiting to happen.”
That is a tricky and very unique interchange Angela. Before we address that U-turn, let’s talk about why that U-turn is even possible. The Alameda/I-225 intersection is an offset SPUI. SPUI is short for Single Point Urban Interchange. According to the Transportation Research Board, the SPUI was developed to improve traffic capacity while requiring less right-of-way than the more traditional diamond interchange. A diamond interchange has the exit and entrance ramps on either side of a bridge like the interchange just down the road at I-225 at Mississippi.
The configuration of a typical SPUI allows for turning movements to be executed in one central area that is either on the overpass or underpass. An example in metro Denver would be the interchange of I-25 at University or C-470 at Morrison Road. According to the engineering company SEH, the first SPUI sprang up in 1974 in Clearwater, Florida.
Since the single interchange point on Alameda at I-225 is on the east side of the bridge and not in the center, it is referred to as an offset SPUI. CDOT engineers designed it that way to allow for enough distance between the existing signaled intersection of Almeda at Potomac and the relatively new interstate interchange. SPUIs were a popular choice for traffic engineers for many years because they increase capacity and therefore can accommodate more vehicles compared to conventional diamond interchanges. Since a SPUI has one signalized intersection, it allows for a simpler phasing sequence for signal control. This also makes it easy for a SPUI to be coordinated with upstream and downstream signals.
The design of the unique interchange is what allows you to make that U-turn. However, that U-turn that you are doing Angela can be a dangerous move. While it is possible, and according to Aurora police not an illegal turn, it is not advised. Aurora Police tell me while there aren’t specific signs saying that a U-turn is not allowed, there is a sign that does say that drivers are only allowed to turn left. Additionally, there is a right-hand turn green light for the cars traveling on eastbound Alameda to turn onto southbound I-225.
The turn is a bit tricky because you first have to be in the far left lane as you come onto the exit ramp. Then, as the green left turn arrow comes on and you start to make that U-turn, you have to be mindful of any eastbound drivers on Alameda who might make a right turn onto that ramp as the same time as you. As you referenced, it is an accident waiting to happen. Aurora police tell me while you should always proceed with caution through an intersection, green light or not, they believe since the sign says left turn only, you could ultimately be at-fault for making an improper turn if involved in a wreck.
One officer suggests making a regular left onto Alameda, turn left onto Potomac and use the parking lot at Potomac/Alameda as a way to get going east on Alameda and to the southbound I-225 ramp. Another easy way is to take a right on Alameda then the quick right on Abilene Ct next to the old Village Inn. Then a right on Abilene Street. Take that to Mississippi, go right and then turn left on the southbound I-225 ramp.
Incidentally, the Alameda bridge over I-225 is relatively new and is vital to moving traffic in the growing City of Aurora. The I-225 interstate spur was originally constructed without an interchange at Alameda. It was November 1997 when voters approved the construction of a $20 million interchange. It was December 1999 when construction of the second phase of the I-225 Alameda interchange project began and completed in March 2001. It was 2002 when the new interchange at Alameda and I-225 opened to traffic.
Denver7 traffic anchor Jayson Luber says he has been covering Denver-metro traffic since Ben-Hur was driving a chariot. (We believe the actual number is over 20 years.) He's obsessed with letting viewers know what's happening on their drive and the best way to avoid the problems that spring up. Follow him on Facebook, Twitter or Instagram or listen to his Driving You Crazy podcast on iTunes , Stitcher , Google Play or Podbean.