Monday, December 12, 2016

Another Perspective on Road Diets

Mid-Block Conflict Points for
Four-Lane Undivided Roadway
versus Three-Lane
A few posts back, I wrote about a road diet that’s being proposed for an arterial into my town, a well-traveled route into the heart of downtown used by both residents and visitors.  While not without its negatives, I think the proposal is reasonable, although other don’t agree.  Today, a friend is going to take over this space to write about the traffic engineering behind road diets, engineering that applies directly to the Petaluma Boulevard South proposal.

Bjorn Griepenburg is a Petaluma native, recently returned to his hometown after an academic career that included stops at UC Santa Barbara and the University of Oregon, concluding with a Masters in Community and Regional Planning.  After a year with Muni in San Francisco, he’s recently become the Policy Director for the Marin County Bike Coalition.

Here’s Bjorn:

Correcting Misconceptions about the Road Diet

Petaluma Boulevard South does not work well for anybody.  In its current configuration, it has four travel lanes and two parking lanes—about 60 feet worth of lanes—crammed into a 52 foot space.  It’s not uncommon to see cars parked on sidewalks or driving between the two travel lanes due to the constrained space.  The pavement is in deplorable condition.  Most importantly, it’s an uninviting and unsafe corridor for people traveling by foot and bike.

Luckily, there exists a well-established solution to all of these problems: the road diet.  Road diets have become the silver bullet of transportation engineering across the country, correcting four-lane roads like Petaluma Boulevard by eliminating a through lane in each direction and replacing them with a bi-directional center turn lane--improving safety for all users, and doing so with negligible impacts on traffic.

With the number of travel lanes dropping from four to three, a common question raised by skeptics follows some form of the following: how doesn’t dropping a lane lead to increased congestion?  I’ll do my best to explain why and how, calling out benefits of the reconfiguration along the way.

The Problem with Four Lanes – and the Case for Three

Petaluma Boulevard South’s four lanes don’t benefit anyone, drivers included.  Studies of road diets suggest that corridors with average daily traffic (ADT)—the number of vehicles that traverse a corridor in a given weekday—of 20,000 or below are strong candidates for road diets.  (

That’s because there simply isn’t enough traffic to necessitate having multiple lanes in both directions. Petaluma Boulevard South falls far below this threshold, even under future build-out scenarios. Even if you head to the Boulevard in its peak hours, you’d be hard-pressed to find the corridor congested with cars clogging up both lanes in either direction.

Crossing and Through Traffic
Conflict Points at Intersections
for a Four-Lane Undivided Roadway
versus Three-Lane
One of the instances in which you will see cars backed up in a single lane on Petaluma Boulevard South is when a vehicle signals for a left turn and must wait for oncoming traffic to clear.  At those times, the cars behind will either queue up behind the left-turning vehicle or pass on the right. The road diet creates a designated space for left-turning vehicles, preventing these backups and the dangerous weaving patterns that they often induce.

Major-Street Left-Turn Sight Distance
for Four-Lane Undivided Roadway versus
Also, as anyone who has turned left off of the Boulevard or crossed it by foot, bicycle, or vehicle can attest, the presence of two lanes in each direction can dangerously obstruct sightlines.  In my daily crossings of the Boulevard at G Street, a far-too-common occurrence involves a driver in the nearest lane stopping to allow me to cross, only to have a car speed by them in the second lane, unaware of my presence in the crosswalk. In the three-lane configuration, those crossing or turning left off of the Boulevard only have to worry about one car in each direction, dramatically improving sightlines. The Federal Highway Administration illustrations above
and to the right highlight these and other conflict points reduced by the conversion.

With each conflict point removed, the chances of a completely preventable tragedy are reduced. Studies have found crash reduction rates ranging from 20 to 50 percent along corridors similar to Petaluma Boulevard South after implementation of a “4-to-3” road diet.  (

Even if the road diet had negative traffic impacts—and it doesn’t—it would be absolutely ridiculous to argue against a project that makes a corridor safer for people walking, bicycling, AND driving. It’s time to start honoring our commitment to public safety through road design. In the case of the road diet on Petaluma Boulevard South, it’s a no-brainer.  – Bjorn

My recent post on road diets triggered extensive comment chains on both Facebook and Nextdoor, a conversation in which Bjorn participated.  I didn’t have the opportunity to join the discussions, but will use my next post to respond to a few of the comments.

As always, your questions or comments will be appreciated.  Please comment below or email me.  And thanks for reading. - Dave Alden (

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