# Eliot Deviation Index

What if there was a measure of how straight a transit line's routing is? How direct is this routing? I had those questions on the afternoon of May 3, 2022. Those were the fundamental questions behind the Eliot Deviation Index - the measure that answers those questions. Put simply, the Eliot Deviation Index of a transit route is the ratio between the line distance and the displacement.

Definition
What exactly is the Eliot Deviation Index and how is it calculated? The measurement is calculated by adding together the straight line distances between each stop, and then dividing it by the displacement of the route. The calculator uses the Haversine formula and spherical trigonometry to determine distances.

In other terms (thanks Mark for this explanation!) - say you've got a transit route with a series of stops - call them A, B, C, D and so on. Calculate the straight-line distance between each successive pair of stops - so A to B, then B to C, C to D, etc. Add all these stop-to-stop distances up. Now calculate the straight-line distance between the first stop and the last stop on the route - say A to Z. The EDI for the route is the total of the stop-to-stop distances (A-B-C-D-...), divided by the first stop - last stop distance (A-Z). So where all the stops are in a nearly straight line, the EDI will be close to 1; where the start and end are close together, but the route between them is long, the EDI will be large.

If the agency or route that you are looking for does not exist yet in the database, don't panic, just let us know. Contact info can be found on the bottom of this page.

Eligibility
Not every transit line is eligibile for an Eliot Deviation Index. There are six criteria that all routes must meet.
1. The line must begin and end at different stops. No loops allowed.
2. The line must have at least three stops. (The Chilmark Rule)
3. The line must not end along the return journey to the beginning.
- 3a. Lines that end with turning loops end at the midpoint of the loop. (The Davis Rule)
- 3b. Lines must travel between two distinct terminals. (The Naugatuck Rule)
4. The line must look like a transit line and actually make sense. (The Quincy Rule)
5. The line must be a currently operating service pattern at the time of calculation. (The Pawtucket Rule)
6. The line must travel primarily by land. (The Hudson Rule)

These rules are based off of eligibility case decisions, which can be read in full here.

The Team
This would have not been possible without the help of some friends. The team currently consist of about a dozen members, and we all enjoy discussing some fun outliers and rural and smaller bus systems where these routes can be found. This is the group of people who like to challenge the eligiblity rules above, and a number of these rules would not be the way they are without them. Join our Discord server today!

Try it Yourself
Want to see the magic of mathematics in action? There is a button at the top of this page that says "Calculator", which will bring you to the online calculator, where you can check out detailed stop listings for any route that has an officially calculated Eliot Deviation Index. You can also use the stop listing on this website to create a custom route and see what the Eliot Deviation Index would be.

Mapping
As of February 2023, KML data is provided for all lines stored in the database here. This can be used in programs such as Google Earth and QGIS.

Measurement
Curious to learn more about how routes are measured, or how to contribute to the database? A guide is available here.

Special Thanks
The Eliot Deviation Index could not have made it this far without some help.
Thanks to those first interested in the measurement in May 2022, when there were only 300 stops in our database.
Thanks to those who questioned the definition of this measurement, in order for me to further clarify what is eligibile and what is not. Not every bus route is the same.
Thanks to the team at my high school who allowed me to tell the story of the Eliot Deviation Index to the rising seniors in the class of 2023.
Thanks to the TransitCon team for allowing me to present on this at TransitCon in April 2023.
Thanks to all of my friends who keep me going with this passion project, as well as those who have contributed to this database in many ways, shapes, and forms.

Contact
Ben Chase (the creator of this whole thing) - [email protected] / @itsbenchase