We need to check the numbers

by Prof. Al Bartlett

August 1, 1999

The numbers in the Denver Post's front page story "More Lanes, More Traffic" (June 28, 1999) raise three serious questions.

1) The story indicated that the present six-lane road (I-25) can carry 256,000 vehicles a day "if it is not widened." Is this reasonable?

Let's do some arithmetic. This is approximately 43,000 cars per lane each day, or an average of about 1780 cars per lane for each of the 24 hours in a day. This is about one car every 2 seconds in each of six lanes. One can imagine cars at this density during the rush hours, but it is hard to imagine this density 24 hours a day. Have I made a mistake or are the numbers wrong in the story?

2) What is the public cost of the widening of I-25 per car accomodated and per person transported?

The Post story said that widening would cost $650 million and "would bring 50,000 more cars a day onto that highway by 2020." As far as I can tell, the 50,000 additional cars would consist of 25,000 cars traveling to work in the morning and then returning home in the evening, so that each commuting car is counted twice each day. If there are an average of 1.2 people per car, the added 25,000 cars that are accomodated by the lanes added to I-25 will get 30,000 additional people in to work in the morning and home in the evening. To do this, it is proposed to spend $650 million of public money. This is $26,000 for each commuting vehicle or about $22,000 for each commuting person. This is an enormous public subsidy for private personal transportation! This is just the initial capital cost: maintenance is extra.

While the state and the federal government [i.e., taxpayers] will pay the cost of the added lanes on I-25, the local communities [i.e., taxpayers] will have to pay for widening of local streets to get these extra cars onto the new lanes on I-25 and then pay for widening streets to get them off the new lanes. At the destinations one needs 25,000 new parking spaces to accomodate all of these additional cars. These extra costs could easily double the $26,000 cost per car of the added lanes, raising the total cost of the public subsidy to $50,000 or more per car accomodated on the lanes that are proposed to be added on I-25. This should be of great interest to fiscal conservatives, because we are talking about big money!

3) The Post's story told of debate over the projections for the future growth of traffic on I-25. Are the projected figures reasonable?

The quoted increase from 256,000 to 306,000 vehicles per day by 2020 after the six lanes are increased to eight, corresponds to an annual average growth rate of only 0.85%. The total expected growth of the commuting traffic is certainly greater than this. A front page story in the Post (June 30) gave figures from which one can calculate that, in the last eight years, the average annual population growth rate of Parker is about 12.9% and of Castle Rock is about 6.6%. The following table shows how a traffic load of 256,000 cars a day in 1999 will grow in the years up to 2020 for 1%, 5%, and 10% annual growth rates.

Apparently the two lanes added to I-25 will take 50,000 daily car trips out of the surrounding neighborhoods and this will fill the enlarged I-25 to capacity, but will it relieve the congestion on I-25? One planner made the understatement, "[Even with the new construction] Congestion [on I-25] isn't going to improve dramatically." If we take 50,000 daily car trips out of the surrounding neighborhoods and put these cars on the enlarged I-25 , how long will it take for population growth to generate 50,000 new daily car trips which then will have to go back into the surrounding neighborhoods, leaving the neighborhoods no better off than they are today?

The total daily car trips could grow from 306,000 to 356,000 in 15 years if the annual growth rate of cars is 1%, 3 years for 5%, and 1.5 years for a growth rate of 10%. It was said that the added lanes will not reduce congestion on I-25. With continued population growth, the added lanes will not reduce the burdens the surrounding neighborhoods suffer because they have to accomodate the spillover traffic from I-25. So where's the gain?

As quoted in the Post's story, Representative Todd Saliman understands the problem. He observed that the projected traffic numbers are probably low. He said, "I know that people wish we could just add a few lanes and our problems would be solved forever... But that's just not how it works."

The general conclusion is, that as long as population growth continues in the Metro Denver area, adding more lanes on major thoroughfares will not to alleviate traffic jams, it will only enlarge the traffic jams.

Unless I have made a dumb mistake in my figures, I think the numbers given in the story in the Post are not reasonable.