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:
http://blog.oregonlive.com/commuting/2013/02/portland_traffic_stressful_and.html
The full report can be found here:
http://mobility.tamu.edu/ums/report/
Transportation Problems and Policy - class blog for USP 456/556 at Portland State University
I would be interested to see how the urban growth boundary affects this issue. Along with this, if the addition of space alongside freeways help to alleviate the traffic congestion associated with accidents. I know certain corridors lack these spaces such as I84
ReplyDeleteThis is a fascinating bit of data-crunching, thank's Glen for sharing it.
ReplyDeleteThe Texas Transportation Institute's Urban Mobility Report is the same ongoing study that's mentioned in this week's reading (Downs Chapter 3: How Bad is Traffic Congestion?). The reading uses data from the 2002 report, and while several of the critiques presented in the reading are still valid (such as the largely hypothetical nature of the cost measures presented) this year's report is much better in a very important way. This year, the report uses actual data gathered from the major roadways by Intrix as the basis for evaluation. Previously, the report was based on modeled performance - this is one of the major criticisms presented in Downs chapter. This year's data, presumably, incorporates any positive effects that better roadway management and increased transit access provide.
Another element of the report that I found interesting was that, on aggregate, streets have more delay than freeways. Possibly some of this may be "spillover" delay, congested streets serving as extra staging space for congested freeways. Additionally, this might be an indication that hierarchical street design (local collectors funneling to major arterials) is making our streets more and more like freeways. I'd be intrested in seeing how the travel time reliability of streets compares to freeways, since I would imagine that the more resilient network design of streets allows for better mitigation of disruptive events like collisions.
@Shawn:
The report suggests that the urban growth boundary is a contributing factor to the congestion and unreliability of commute times in Portland. By encouraging compact development, the UGB also increases congestion. However, it also leads to shorter distance trips and safer roadways.
The report uses some basic roadway design elements to estimate the amount of the traffic congestion that's due to incidents like collisions, but that's about all:
"The basic Urban Mobility Report methodology includes an estimate of the delay due to incidents. This estimate is based on roadway design characteristics and incident rates and durations from a few detailed studies. These give a broad overview, but an incomplete picture of the effect of the temporary roadway blockages." (pg B-5)
They do report that active incident management can have a very beneficial effect on congestion, since over 50% of most delay is caused by incidents. Using techniques such as monitoring cameras and direct connections with police (picture the room without windows from "Bureaucrats"), these programs can have benefit/cost ratios of up to 10:1.
It is quite interesting that policies creating shorter commuting distance triggers a low commute reliability. Then, longer commute distance will improve commute reliability? Well, it seems to be not a good approach. That is because amount of vehicles on road will not be decreased, but the travel distance will be increased. It would be likely true that the situation got worse because of more traffics from other alternative mode users(transit, active transportation) who want to shorten commute time by switching current modes to automobiles. I am wondering dense land use development will reduce amount of automobile traffic on roads? If yes, how dense it should be in order to see dramatic(?) reduction of automobile uses?
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