Thursday, August 17, 2017

Diverging Diamond Interchanges, Complete Streets and the 710

In July, Caltrans released its Recirculated Draft Environmental Impact Report (RDEIR) for the I-710 Corridor Project. This document follows the 2012 DEIR and provides updated alternatives meant to incorporate community concerns about the agency’s freeway “modernization” plans. In addition to anticipated federal and state funding, Metro has allocated over $1 billion in Measure R and Measure M funding to the project, which envisions adding new general purpose or low-emission truck lanes to the interstate corridor.

According to the RDEIR, modernizing the freeway will also include redesigning the way the freeway interacts with local roads throughout the corridor. In order to address interchanges it calls “outdated,” Caltrans recommends replacing many of them with what are known as diverging diamond interchanges, or DDIs. Major roads from Anaheim Street in Long Beach to Florence Avenue in the city of Bell could receive this treatment, in the hopes that it will help reduce vehicle delays.

Source: Diverging Diamond Interchange Informational Guide. FHWA, 2014.

DDIs have become increasingly popular across the country as they provide a way to accommodate an increased volume of cars while minimizing the need to physically widen the road. The defining characteristic of a DDI is the crossover that directs automobile traffic from the right side of the street to the left as the road intersects the freeway. The interchange functions like two one-way streets, allowing drivers to make a left turn onto the freeway without slowing down or having to wait for a separate traffic signal.

The Federal Highway Administration also considers the diverging diamond interchange to be a superior design because it allows space to accommodate people who walk or bike. DDIs are thought to be beneficial because they reduce the distance people walking have to travel at a time, separating the road crossing into several smaller chunks.

But it is clear that people walking and biking are not the first priority here. The main objective of the design is pushing a marginally greater number of cars through an intersection at marginally greater speeds. Despite that cars are fast and people are slow, DDIs focus on shortening the distance cars have to travel even if it makes traveling by foot significantly more burdensome. For people on foot, there are a number of additional concerns aside from the circuitous routing. The unusual traffic patterns can lead to confusion about which direction is safe to cross, and the structure of the median walkway can impair the ability of drivers to see people who are walking. Furthermore, by allowing for cars to travel at a greater speed, it is less likely that drivers will yield to pedestrians where there are unsignalized turn lanes.

At Longbeachize, Brian Addison brings needed context to how the 710 divides the city of Long Beach, and what it can be like for a person to have to use roads that were built without them in mind. Despite the attempts at rote inclusion of complete streets elements, the DDI unfortunately replicates road design elements that repel people who walk or bike, and make the experience less pleasant and safe.

Source: I-710 RDEIR. Caltrans, 2017.

This diagram shows Pacific Coast Highway in Long Beach as it exists presently. It is an extremely abrasive environment that lacks even a basic sidewalk on the north side of the street. But where the sidewalk does exist, even though people can and do rely on that connection, it’s not enough to make the street pedestrian-friendly.

Source: I-710 RDEIR. Caltrans, 2017.

The baseline for the PCH and similar major roads along the corridor is so low currently that it is tempting to say that the DDI solves a lot of existing problems here. After all, it provides space for people walking or biking in either direction, and introduces a number of traffic signals where there currently are none. However, here and elsewhere, the reconfiguration of the interchange is being used to add a third lane for through-traffic in each direction. The de facto road widening will make these streets look and feel more like freeways, enabling drivers to travel at faster, more dangerous speeds. People walking and the differently-abled will have to walk out of their way into the center of the road, surrounded on either side by cars moving at near-freeway speed. It is a claustrophobic and threatening environment.

A true complete street, providing access and mobility to road users regardless of the mode they choose, is not one that gives leftover space to people biking or on foot once the goal of maximizing automobile throughput is achieved. In Los Angeles, more than anywhere else, it should be obvious that the cumulative impact of this mindset is devastating to communities.

Residents of the cities along the 710 corridor experience high rates of poverty and unemployment. The interstate itself is dangerous both as a major source of hazardous pollutants and as a physical barrier within the built environment that limits the abilities of residents to safely access their own communities. It is unfortunate, given this opportunity to “modernize” the freeway, that people who travel by foot or by bike are still receiving less consideration than small time-savings for drivers.

 


4 responses to “Diverging Diamond Interchanges, Complete Streets and the 710

  1. It’s easy for you (or anyone) to complain, but I didn’t read any discussion of an alternative design. Please diagram what you want to see constructed and explain why it’s better.

    — MarkB


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