The best explanation I have heard, is that they believe that slower traffic is better for the environment, and so they try to slow everyone on the freeway. This combined with the fact that all the limits are 10mph slower than anywhere else in the country, and it's a recipe for disaster. Lots of accidents.
Alright. Enough Portland bashing
! Besides AMG's claims that all Portlanders are passive aggressive, Birkenstock wearing, Volvo driving, Whole Food shopping, seed eating, environmental weirdos........ there are other slightly more scientific explanations. Let me offer another perspective. Human Factors Engineering and Urban Planning reasons. AMG, if you've spent a lot of time in Portland then you have seen the 2 inch iron rings fastened to the curbs. Those rings still remain from the days when Portland commerce was carried by horse drawn wagons.
NickH, the rings remain on downtown curbs in case you need to tie up your horse. (no joke)
Portland traffic planning engineers were way behind many other cities in urban growth boundary and road designs. They are much better at design now, but they, and Vancouver, have spent years rebuilding their infrastructure to match standards developed by other cities. For example, cramped tiny diameter circular freeway on-ramps used to provide almost no merge time, unless you were driving a sports car that could do zero to 60mph in 4 seconds (if not, drivers had to sit for days and wait for an opening in freeway traffic). Those and other covered-wagon era design flaws hindered drivers and driver development.
Some of their city bridges are very narrow because they were designed for Model T's.
Sandy River Bridge built 1912 - similar to some downtown Portland Bridges in width
Charming, but narrow (Sign says - "
One Way Traffic for trucks and buses", but I know from experience that it's tight even with an SUV)
The countryside outside Portland has many many covered and other early century bridges (not something you see outside Los Angeles.)
So part of the problem Portland drivers had was caused by old road design human factors issues, many of which are not apparent to the general public. They've installed a state of the art light rail system that is taking some of the burden off the roads, and the freeway on/off ramps are being redesigned, along with slowly replacing the old poor road signage.
The new
Max Light Rail System
.
If someone wants to see a well engineered road system, they can move to Los Angeles. But Portland and the surrounding countryside has its charms.