A NEW DIAMOND FOR AUSTIN

By Kevin Wilcox

June 7, 2016—The Texas Department of Transportation (TxDOT) and its consultants are completing the engineering design of a 2.069 mi, $53-million project that will alleviate congestion on Loop 1 on the west side of Austin—known locally as the MoPac Expressway, for the Missouri Pacific Railroad—a critical north-south artery in the state's capital. To accomplish this, the department plans to use both a traditional diamond intersection at one point (on La Crosse Avenue) and an uncommon diverging diamond intersection (DDI) at another (Slaughter Lane).

The DDI is still a relative rarity in the United States, though it is gaining popularity because of its efficiency and safety. The first DDI was constructed in 2009, and in the following six years, approximately 59 others were completed. In a DDI, motorists are diverted to the left side of a bridge as it passes over the main lanes and diverted back to the right after the intersection (see the green arrow in the illustration below). One benefit of this arrangement is that the southbound frontage road traffic (shown by the orange arrow at the top left of the illustration below) will not need to stop when the traffic following the green arrow has a green light. Also, the green arrow traffic crossing the expressway will have a signal-free left turn.

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