(October 7, 2014) – A Portland Bureau of Transportation analysis of 20 years of traffic data found that road diets, or reconfigurations, on Portland’s streets lowered speeds and significantly reduced traffic crash rates.
The results demonstrate the value of these reconfigurations as a tool to improve safety on busy multi-lane streets.
|Portland Road Diet Evaluation: 1994 - 2014
||Project Length (mi)
||Before Data Range
||After Data Range
||% Change in Crash Rate
||% Volume Change
||% Change in 85% Speed
|NE Glisan, 22nd - 32nd
|SE 7th, Division - Washington
|SE Tacoma, 6th - 11th
|SE Division, 60th - 80th
|NE Glisan, 60th - 80th
Sources: Oregon Department of Motor Vehicles Crash Data, 1993-2012, Portland Bureau of Transportation (vehicle speeds)
PBOT analyzed crash data on all five road sections in Portland where the bureau reorganized a two-way arterial from four vehicle lanes to two vehicle lanes and a center turn lane. In some instances, Portland also added bicycle lanes. PBOT often refers to these changes as “road reconfigurations.” Nationally, this treatment is known as a “road diet.”
On Portland’s streets that have undergone road diets, PBOT’s analysis found on average:
- 37 percent reduction in traffic crash rates
- 10 percent reduction in the 85th percentile speed*
In addition, the amount of traffic dropped six percent, which mirrors traffic volume reductions across the City since 2005.
The Federal Highway Administration (FHWA) has identified these road reconfigurations as one of nine “proven counter safety measures” to reduce speeding, crashes and fatalities.
Portland’s experience mirrors national data. FHWA cites a 29 percent reduction of traffic crashes nationally on streets undergoing road reconfigurations.
FHWA notes that road diets provide multiple benefits to all road users. These benefits include improved vehicle compliance with posted speed limits and a corresponding decrease in crash severity; reduction of “multiple threat” crossings for pedestrians (one driver stops for pedestrian while driver in adjacent lane does not); reduction of rear-end and side-swipe crashes; increased opportunity for on-street parking, and improved safety for bicyclists when bike lanes are added (which also provide an additional buffer for pedestrians).
In 2013, PBOT implemented road reconfigurations on SE Division between 60th-80th avenues and NE Glisan between 60th-80th avenues. Both segments saw more than a 50 percent decrease in vehicles driving over the posted speed limit (see links). PBOT anticipates an accompanying reduction in crash rates, although crash data is not yet available.
Over 90 percent of Portland traffic crashes take place on busy streets. PBOT’s High Crash Corridor program employs short-term, low-cost treatments to improve safety on these roadways. Given their high cost-benefit ratio, road reconfigurations are an important tool in that program’s tool box.
*The 85th percentile speed is a value that is used by many states and cities for establishing regulatory speed zones. It is the speed that 85% of vehicles are observed traveling at or under during free flowing conditions.