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Interchange Index

I-10, AZ 51, and AZ 202
Phoenix, AZ

"The Mini-Stack"
Overhead Picture
Overhead Picture from terraserver-usa.com

Overview: Interstate 10 (the Papago Freeway) runs east-west through Phoenix; in this interchange, it actually enters from the west and leaves to the south. AZ 51, the Squaw Valley Freeway, runs northward from here; the freeway to the east is AZ 202, the Red Mountain Freeway. The bulk of downtown Phoenix lies to the southwest of this aerial photo. Two local streets also interact with the freeways present here: North 24th Street runs north-south, parallel to and east of AZ 51; and East MacDowell Road runs parallel to and north of AZ 202. This interchange is Exit 147 on I-10, and is the terminus of AZ 51 and AZ 202 and, as such, is unnumbered on both freeways.

Interchange Description: This interchange is known as the "mini-stack" interchange; it is well named, as it is basically a variation on the typical four-level stack interchange. Here, however, instead of two major highways meeting at right angles and crossing each other, there is one major highway (I-10 in this case) which makes a right-angle turn, and two other freeways that branch off of it at the same location. It bears a number of similarities to a standard four-level stack; however, the main highway travels through a right-angle turn, and all entrances and exits from this highway are on the right side. (Note that the roadway which rises out of the median of I-10 to the west and leads straight into AZ 202 to the east is actually a direct exit/entrance from the I-10 HOV lanes; this is naturally an exception to the previous statement.) Each offramp is semi-directional (the ramp to AZ 51 is always to the left of the ramp to AZ 202), and the splits at the end of AZ 51 and AZ 202 are also semi-directional. (From AZ 51 SB, continuing SB is the left half of the split; the right half then splits between I-10 WB to the right and AZ 202 to the left.)
Two other interchanges overlap the junction of I-10, AZ 51, and AZ 202. These are both SPUIs (single point urban interchanges), which are an effective way of getting traffic on and off of a local street without any extra grade separation. One is at the intersection of AZ 202 and North 24th Street, and the second is at AZ 51 and East MacDowell Road. Notice that the ramps from these SPUIs into the main interchange don't provide full access, however; they only traffic to reach North 24th Street from the west, and East MacDowell Road from the south. Really, though, this isn't a problem at all. Traffic from the north or south trying to reach North 24th Street can just use the AZ 51 SPUI at MacDowell Road and travel east a few blocks to 24th Street, or even use another interchange further north or south. The reverse is true for traffic trying to reach MacDowell Road from the east or west.

Comments: In my opinion, the mini-stack is a very effective way of handling traffic at a four-way junction, where most of the traffic is making a right-angle turn. Granted, the sharp turn on I-10 probably causes trafffic to slow down, but it is much better than having to use a one-lane ramp through a standard stack interchange or (shudder) a cloverleaf loop ramp. The lack of left exits or slower loop ramps is appealing, and makes the interchange easier for motorists to navigate.


Links
  • I-10 at arizonaroads.com


Corrections? Suggestions? More information is always welcome.
Suggestions for more interchanges to cover on this site are great too.
Contact the author, Dan (known as DanTheMan on misc.transport.road):
twowheel@email.com