A new city study shows the big payoff the city has quietly seen from a few uses of one of the least-understood tricks in traffic engineering: the 4-3 road diet.
Converting four general travel lanes to two plus a turn lane and (in some cases) painted bike lanes have prevented about 525 crashes on three Portland streets — Northeast Glisan from 22nd to 32nd; Southeast 7th from Division to Washington; and Southeast Tacoma from 6th to 11th — during the 16 years studied, the analysis released this week found. The number of traffic crashes on those streets dropped 37 percent.
Traffic volumes on those three streets, meanwhile, fell only by an average 7.7 percent, suggesting that the safety and access improvements weren’t accompanied by major new burdens on drivers’ mobility.
The number of crashes being prevented on each of those streets, of course, continues to rise: by about 37 more every year among the three of them.
Gabriel Graff, operations and safety manager for the city’s Active Transportation Division, said that though the benefits of 4-3 road diets are “counterintuitive,” they ultimately work for a simple reason: people rarely have either a temptation or a reason to pass each other on a city street.
On streets with two lanes in each direction, Graff said, “the people who are going fast have the opportunity to kind of weave between the two lanes. That results in sideswipe crashes.”
Letting left-turning cars sit in the center turn lane, meanwhile, prevents traffic backups and preserves road capacity.
In city jargon, the projects are sometimes referred to as “road reconfigurations.”
Graff said the price of all five road diets considered in the city’s analysis was “in the $100,000 range,” or up to $120,000 or so for projects that added new median islands or other improvements.
“The cost/benefit is really high,” he said. “For the cost of one improved crossing — a median improvement or rapid-flashing beacon that provides a point improvement, you can reduce crashes across 10, 20 blocks.”
In addition to the three road diets listed above, all of them completed between 1997 and 2003, the city looked at the results of two 2013 road diets: Northeast Glisan and Southeast Division, both between 60th and 80th avenues. Though those projects are too recent to show reliable crash data, the city did determine that typical auto speeds fell by 9.8 percent and traffic volumes fell 4.4 percent.
The road diets on inner Glisan, inner 7th and Division all added bike lanes, increasing street capacity and, in the case of 7th, creating one of the central east side’s most important north-south bikeways. The outer Glisan and Tacoma road diets added auto parking.
Graff said road diets have become a standard tool in cities across the country and are now being enshrined in federal policy. Last month, Transportation Secretary Anthony Foxx announced that the Federal Highway Administration is preparing to release a national guide to instituting road diets.
“When we’re on our game, we’re able to combine a really significant safety improvement with some paving maintenance,” Graff said. “So when you’re resource-constrained like we are, that’s a big help.”
You can read the city’s full analysis and dig into some of the numbers behind it on the city’s website.