Impresa Clients

Thirty Seconds Over Portland :

A Comment on the “Costs of Congestion” Study

Joe Cortright, Impresa,

April 20, 2006

Is it worth spending an additional $6 billion on transportation to save an average of 30 seconds on the typical trip 25 years from now?

The “Costs of Congestion” report estimates the economic benefits from improving Portland ’s transportation system over the next two decades that would result from spending an additional $6 billion (that is, increasing projected transportation spending from $4 billion to $10 billion). These travel time savings account for nearly all the net economic benefit of the project.

How much time will the typical traveler save? The report itself doesn’t actually say, but it is possible to compute the answer from the data contained in Table 4-4 of the report, reproduced below. (The rows labeled “MPH” and “Miles Per Trip” are taken directly from the report; I’ve calculated that values in the rows labeled “Minutes per Trip” and “Savings/Trip” as explained below.)

Table 4-4. Improved System Scenario: Impact on Future Road Traffic
(Costs of Congestion Study, Page 38)

		Type of Trip
Indicator	Scenario	PM Peak	Daily Total	Cars	Trucks

MPH	2000 Current	29.2	30.7	30.5	39.3
	2025 Planned	23.8	27.3	27.0	33.0
	2025 "Improved"	25.7	28.8	28.5	34.5

Miles Per Trip	2000 Current	6.6	6.4	6.1	19.3
	2025 Planned	6.5	6.3	6.0	19.3
	2025 "Improved"	6.6	6.4	6.1	19.1

Minutes per Trip	2000 Current	13.6	12.5	12.0	29.5
	2025 Planned	16.4	13.8	13.3	35.1
	2025 "Improved"	15.4	13.3	12.8	33.2

Savings/Trip (Seconds)	2025 “Improved” Minus 2025 Planned	58.7	30.8	29.5	112.4

The three rows labeled “MPH” give the estimated average travel speed in miles per hour for 2000, for the projected 2025 planned improvements and the speed with an improved system in 2025. The rows labeled “miles per trip” show the average number of miles for each type of trip. The next three rows—“minutes per trip” are calculated by dividing the number of miles of each trip (6.1 for current car trips) by the average speed (30.5 miles per hour) and multiplying the results by 60 to get the number of minutes for each average trip. So the average car trip in 2000 takes about 12 minutes.

The final row, savings per trip, is computed by subtracting the travel time for the “planned scenario” from the travel time for the “improved scenario” then multiplying the result by 60 to estimate the time savings in seconds. So, for the car trip, the planned scenario is 13.8 minutes and the improved scenario is 13.3 minutes for a savings of .5 minutes, or a little less than 30 seconds.

In sum, Costs of Congestion shows the following:

Congestion will increase -- No matter what we do, travel times will increase from current levels; today's average 12 and a half minute trip will to about 13 minutes and 48 seconds (with the “Planned” level of investment) or to about 13 minutes 28 seconds (under the “Improved” scenario).

$6 billion = 30 seconds -- Compared to the “planned” level of investment, spending an additional $6 billion will reduce the average car trip by about 30 seconds, reduce the peak hour trip by about a minute, and will reduce the average 20 mile truck trip by about 2 minutes.

Shorter average trips would produce the same result -- It’s worth noting that a reduction in average trip lengths of about one-quarter of a mile would achieve exactly the same time savings. (At an average system speed of 30 miles per hour, it takes about 30 seconds to travel one-quarter mile). Changes to land use patterns--including greater density, better jobs-housing balance, more mixed use development and stronger town centers--or people moving closer to where they work (or working closer to where they live) could achieve the same time savings (and associated economic benefits).

Alternative investments might have greater benefits -- Before spending $6 billion on additional transportation investments, we might want to consider what the economic benefits of alternative investments, such as increasing the quality of the region’s higher education or K-12 education.