Thursday, June 20, 2024

Cantabrigians may recall the famous thought experiment by physicist Erwin Schrödinger in which an imaginary cat is placed in an imaginary box, unobserved, with a vial of deadly poison and an unstable radioactive trigger. Without observation, we cannot know whether the cat is alive or dead. From a quantum mechanics standpoint, we might come to the conclusion that the cat is simultaneously alive and dead – so long as it remains unobserved. The moment we open the box to make an observation, reality asserts itself. The cat must either be alive or dead; it cannot be both.

Duality, of course, is common in nature. Light can be a wave; it can also be a particle. Peanuts are healthy for many; a deadly allergy for some. And bike lanes, it seems, can be safe and unsafe simultaneously, according to various analyses promulgated by Cambridge residents as well as other users of our roads. How can these seeming contradictions coexist?

Some of the confusion has been caused by simple errors in data analysis. The cycling advocacy group Cambridge Bicycle Safety recently wrote (“It’s no surprise Cambridge roads are getting safer,” May 22) claiming that Cambridge crashes resulting in EMS transport to the hospital had decreased by nearly 40 percent since 2016. Independent analysis shows this claim is factually incorrect. 

According to the Cambridge Police Department crash log last updated May 31, there were 201 crashes in 2016 resulting in one or more hospitalizations, compared with 203 in 2022 – almost identical, and a far cry from “stark reduction” alleged by Cambridge Bicycle Safety. In fact, crashes resulting in hospitalization have averaged 205 each year since 2016, with very little variation (standard deviation ±4.2) aside from the pandemic years of 2020-2021. 

Armed only with Microsoft Excel and an internet connection, any private citizen may verify these numbers for him or herself simply by downloading the latest version of the Cambridge Police Department Crash Log in CSV format; sorting by column J (hospitalizations) and deleting any entries of 0 to filter for only crashes with 1 or more people transported to the hospital by EMS; using the YEAR function to parse the year of occurrence from Column A (Date and Time); and finally using the FREQUENCY function to create a frequency table of crashes per year. This should result in the graph above, which unfortunately bears minimal resemblance to what was shared in Cambridge Day by Cambridge Bicycle Safety advocates:

Despite this data analysis flub by Cambridge Bicycle Safety, the overall goal of cyclist safety is noble and does not deserve to be overshadowed by the mistakes of any individual. Cyclists are an important segment of road users, accounting for nearly 7 percent of Cambridge commuters, according to U,.S. Census Bureau data from 2021. Whether one travels by motor vehicle, bus, train, foot, bicycle, scooter or any other mode of transportation, road safety should be the goal of any reasonable citizen. That’s why it’s so important for the City of Cambridge to investigate our current cycling infrastructure carefully and determine whether safety concerns remain.

At the heart of the bike lane conundrum is the presence of natural variation. Numerous scientific studies have demonstrated a trend of increased cyclist safety with the installation of cycling infrastructure such as separated bike lanes. For every average calculated, though, there is natural variation around the mean – just as genetic variations and environmental characteristics combine to make some of us skinnier or wider than the population average, some bike lane installations will inevitably perform better or worse than others due to the unique characteristics of each locale – road configuration; intersection density; even socioeconomic factors, according to some studies. News reports frequently ignore this nuance, yet measures of natural variation are present in almost all scientific papers. 

For instance, a recent study by the Federal Highway Administration notes that the type of quick-build separated bike lanes being deployed around Cambridge (in which traditional bike lanes are replaced by separated bike lanes with a blend of flexible delineator posts and other vertical elements, such as parked cars) should theoretically reduce bicycle crashes by an average of 36 percent (crash modification factor of 0.640). In the very same data table, however, the researchers were careful to note that the standard error of their estimate was 0.203 – in other words, significant natural variation was observed in bike lane effectiveness, with some percentage of bike lanes expected to actually result in crash increases (i.e., crash modification factors above 1.000).

The natural variation reported by Federal Highway Administration researchers could explain the seeming paradox of recent observations by John Hanratty and the Cambridge Streets For All neighborhood group, who reported that some Cambridge neighborhoods appear to be experiencing higher levels of injury-causing crashes since bike lane installation.

Scientists have identified some (but likely not all) of the risk factors that raise bicycle lane crash rates. For instance, in the same Federal Highway Administration study, researchers determined that bicycle crashes are more likely when bike lanes are installed at locations with mixed land use (for instance, commercial-residential neighborhoods such as Hampshire Street, Main Street or North Massachusetts Avenue). Another recent study in the journal Accident Analysis & Prevention provides additional variables to examine, identifying features such as frequent driveway and road intersections, lack of continuous separation and increased complexity for turning drivers as major risk factors for protected bike lanes. And a 2022 piece in Transport Reviews reported that single-bicycle crashes, which account for 52 percent to 85 percent of cyclist injuries (based on hospital and emergency department data), are most commonly caused by road maintenance issues such as uncleared ice and snow, fallen leaves or potholes.

In the context of natural variation, a location-specific or neighborhood-specific approach becomes key. Whereas aggregated multicity studies provide generalized information on cycling infrastructure effects averaged across a wide area, localized data allows city planners to home in on specific problem areas and come up with neighborhood-specific solutions. The analysis done by Hanratty and Cambridge Streets For All takes some steps toward this approach, but could be significantly refined with additional information such as monthly location-by-location traffic counts for motorists, cyclists and pedestrians. Such data is not available to the Cambridge public, in large part because the city has not collected it.

For a city such as ours, populated by so many world-class scientists, engineers and experts of every stripe, it seems a travesty that we should leave any issue unresolved for lack of data – particularly, an issue so pressing as public safety. Yet today, in Cambridge, it is not the welfare of an imaginary cat that we have failed to check in on, but rather the lives and livelihoods of our own residents, as well as the many commuters, customers, service providers and others who use our streets each day. Quantum superposition does not work at the city level – we cannot remain both safe and unsafe while debating endlessly with incomplete data; we must open Schrödinger’s box, make the necessary observations and collect the necessary information to determine just what is going on on our streets. And from there, we can fix it, because one thing Cambridge is not short on is invention and ideas. We just have to take that crucial first step.

Serenus Hua, Hampshire Street