"If you obey all the rules you miss all the fun." - Katharine Hepburn
Question #92277 posted on 06/07/2019 10 p.m.

Alright Board,

Since you all basically passed on question #91937 when originally asked, I thought I would tee it up again for Alumni Week. I know there are a lot of quantitatively-oriented people in the Alumni group, and I figure they might have fun with it.

----Original Question----

I regularly ride UTA FrontRunner from the North Temple Station to the Provo Station. I do this for lots of reasons, one of which is my perception that I'm reducing the carbon and particulate emission of the trip by taking FrontRunner versus driving. A few days ago, I was looking at the massive diesel fuel tanks on FrontRunner, and it made me wonder how many people have to ride the train in order for the fuel saved from not driving cars to offset the fuel burned by FrontRunner.

Question 1: how many vehicle miles must FrontRunner be taking off the road per FrontRunner mile in order for the train to "break even" in terms of carbon and particulates?

Question 2: Do we have any decent way to calculate, based on FrontRunner trips and ridership data, if FrontRunner, as a whole, is making things net better off in terms of carbon and particulates?

Assumptions: Assume that every vehicle mile is driven by a theoretical "average" vehicle. Assume that nobody is driving to or from train stations.

(I also understand that there are lots of other reasons to have a public transit program. I'm just talking about the pollution aspect here, and am not trying to label FrontRunner as "good" or "bad" on this basis).



Dear G,

So, answering your questions would require knowing several pieces of information namely:

  • The typical output of carbon dioxide and particulates by the engines on Front Runner.
  • How much the engines are being used.
  • The typical output of carbon dioxide and particulates by the average car driving in Utah.
  • How many rides FrontRunner is replacing (This isn't the same as number of riders, because you could have multiple people in a car, or people using the FrontRunner that wouldn't be using a car).
  • Additional adjustments such as fewer cars on the road leading to less idling and less pollution (thanks Zedability for pointing that out).

From there basically you multiple the average output per mile of the FrontRunner engines by the number of miles per day and compare that to the output of the cars that it would be replacing. As much fun as doing all these calculations by myself would be, it turns out that UTA already does similar calculations. The best public report I could find talked about the environmental impact of a "Free Rider Friday" event. The data they have isn't exactly what we need, but we can use it in a pinch to get data that will be useful to us.

According to this report the free fare friday increased ridership by 66.4% to 30016. This means that the average ridership is 18038, so the change in ridership is 11978 people. The report lists the amount of cars taken off the road and the environmental impact for the day, but the report also includes TRAX. The total number of cars taken off the road was approximately 17,560 and the total environmental impact was three tons of vehicle air pollutants and more than 200 tons of greenhouse gas prevented from entering atmosphere. If assume that the impact from FrontRunner versus TRAX is proportional to the amount of increased ridership. TRAX ridership increased by 32.2% to 79,825 which puts average ridership at 60,382 and gives an increase of 19,443 riders. The total percentage of ridership increase in total due to FrontRunner is 38.12% of the total change, which we multiply by the total impact to get the total of 6,694 cars removed from the road and 1.14 tons of particulates and 76.24 tons of greenhouse gases out of the atmosphere per day!

I'm sorry I couldn't answer your question exactly but I hope this answers the bigger question of the environmental impact of FrontRunner. It would appear that FrontRunner is making a positive impact environmentally and has even greater potential as usage goes up. If you'd like to look at the report you can get a better look at the numbers and analysis from the lovely engineers at UTA. Hope this helps!




Dear Jeepers, 

Hey man, I'm just here to prove that the current writers are still totally capable and awesome. I'm not particularly quantitatively inclined... which is why I didn't answer this the first time. But I'm not going to let this one slip past us. So for this answer, Tipperary and I put our heads together to tackle this question. 

First of all, we need to make an assumption: 1 vehicle mile = 1 passenger mile. Because cars can carry fewer people than Frontrunner can, it's easier to calculate things in terms of individual riders, assuming that each person that rides Frontrunner is one fewer person driving. 

Now for the facts: 

The average car emits 0.89 lbs of CO2 for every mile driven (see data found here.) Using the 1=1 assumption, that means Cars have a "per passenger mile" emission of 0.89 lbs. 

Frontrunner will emit X lbs of CO2, even if it's making its rounds with no passengers. The emissions per passenger goes down the more people that ride (logically. X/n will decrease as n increases.) To find where the point of equality is, we need to know how what n equals that would result in a 0.89 lb per passenger mile emission rate. 

Using data from the 2013 Sustainability Report, we charted the relationship between how many daily riders are on Frontrunner and the emissions per passenger mile reported for that year. Here's that chart: 


The points read from left to right, plotting 2009, 2010, 2011, 2012, 2013, and the farthest point extrapolating the emissions for 2018. The red point represents the point of equality that we found, indicating how many riders would have to take Frontrunner daily to equal the same emissions per passenger mile as if they were driving. That number is 3775. 

In other words, as long as there are at least 3775 riders on Frontrunner daily, it "breaks even" in terms of carbon emissions. I couldn't find reliable information on particulates, so we're just going to decide it's the same number (even though it's probably more, since diesel emits more PM2.5 than petrol cars do. But, since there isn't specific numerical information about PM2.5 emissions of Frontrunner, I can't answer that for now.) 

So, is Frontrunner really better for the environment? Based on this data, yes. The most recent data shows that Frontrunner hosts 17,600 riders daily, more than quadrupling the necessary ridership to make up for the emissions. Plus, as technology advances, we think of even more ways to improve emissions, and based simply on the fact that Frontrunner can take more people than cars ever can, it will always be more efficient.

Hope this satisfies your query. 


Guesthouse and Tipperary