According to a study conducted by Texas A&M University and published in December, Portland has the 6th worst commute time reliability in the nation. The Urban Mobility Report found that in order to be on time for 19 out of 20 appointments a 20-minute drive away, the average Portland freeway commuter had to leave 85 minutes early during rush hour.
The study found that these delays were caused mainly by the circular structure of Portland's freeway system and the bottlenecks that this structure caused. One or two well-placed traffic accidents could leave the roads deadlocked.
The study uses the PTI, or the Planning Time Index to calculate the percentage of late trips. For example, to only be late one day a month, the 95th percentile is used and the Portland PTI of 4.26 is multiplied by the non-rush hour trip time (20 minutes in the example above) to get the departure time. In order to only be late one day a week, the 80th percentile Portland PTI is used (2.15) for multiplication. The PTI is also affected by crashes and weather conditions.
The report also estimated that these delays, which equated to 44 hours per person in 2011, cost Portlanders 25 million extra gallons of fuel consumed, almost 52 million wasted man-hours, and total over $1.1 billion dollars. According to a response by one of the authors of the study in the Oregonian, the investment in public transit and bicycle lanes did provide a noticeable advantage over other similarly-sized cities.
A good summary of the report can be found here:
The full report can be found here: