User:TheWhole151/Eliot Deviation Index

The Eliot Deviation Index (EDI) is a ratio created in 2022 by transit researcher Ben Chase to find how direct a public transport route's routing is. The meaasurement was the subject of a session at public transit conference TransitCon in early 2023.

Definition
As a mathematical ratio, the Eliot Deviation Index is defined as:
 * $$i = \frac{d_{(1,2)} + d_{(2,3)} + \ldots + d_{(n-1,n)}}{d_{(1,n)}}$$

where:
 * $$d_{(a,b)}$$ is the great-circle distance between two stops along a route, with $$a$$ and $$b$$ being the order along a route that a given stop falls at, and
 * $$n$$ is the number of stops along a given route.

By this definition, the value $$i$$ cannot be lower than 1.

Rules
There are distinct rules for route eligibility defined, in addition to the above formula:
 * 1) The line must begin and end at different stops. No loops allowed.
 * 2) The line must have at least three stops.
 * 3) The line must not end along the return journey to the beginning.
 * 4) The line must look like a transit line and actually make sense.
 * 5) The line must be a currently operating service pattern at the time of calculation.
 * 6) The line must travel primarily by land.

History
The Eliot Deviation Index was created on May 3, 2022 by transit resarcher Ben Chase after questioning how straight any transit line's routing was. In their original blog post about the EDI, Chase noted that the measurement uses the Haversine formula to calculate the distances between transit stops, and that it is named after a nickname of theirs.

The measurement's database was known to support 185,000 stops around the United States and had 467 calculations made at the end of October 2022, with an increase to 1,575 calculations and 290,128 stops at the end of April 2023.

Development
The EDI is known to be a collaborative effort with others involved in transit planning and related fields.

Application
The Eliot Deviation Index can be used as a comparison between transit routes, providing a common measurement between routes operated in different jurisdictions, and is looked at in context with other planning factors. Chase noted in a follow-up blog post in October 2022 that a higher EDI value is not necessarily better than a lower value.

The measurement has also been used within advocacy groups, with the idea that a more direct route, which would have a lower Eliot Deviation Index, helps people get places faster. Adding a deviation, raising the value, leads passengers to spend longer times traveling, so that someone can be closer served by a bus line. Instead, if agencies ran buses along major corridors, like in many cities, people get places down the line faster. These deviatory routes typically exist in suburban areas, where it is often unsafe to walk from the bus to a final destination, so the bus deviates. We can instead create more direct routes and invest in better first and last mile connections, like bike lanes and sidewalks.