Clickety Clack, Noise There And Back

A practical and reliable long-term approach for a quieter SkyTrain.

The SkyTrain is a core component of the Public Transportation system in the Metro Vancouver Region. The Expo and Millenium lines span over 60 kilometers and have recorded over 80 million annual boardings every year since 2018 [i].
Following many noise complaints from neighbouring residents of the SkyTrain tracks, SLR Consulting Ltd (SLR) was asked in 2018 to conduct a series of tests.
The scope of their work was to quantify and map SkyTrain noise along certain portions of the SkyTrain network [ii]. SLR later submitted a series of recommendations and an implementation plan laid out in their 2021 report.[iii]
After reading SLR's report, we created some tools to help implement one of SLR’s recommendations to conduct "the ongoing monitoring of in-car noise levels each week" and also collect additional data useful for mitigating noise emitted by the SkyTrain.

Data Collection

Over the course of 57 days, we conducted 107 surveys, with each survey lasting an average of 21 minutes.

The dataset contains 78,400 data points representing 38 hours of data covering a distance of 1,610 kilometers on the same portion of the Expo Line.
All measurements were conducted using a Motorola E (2nd Generation) running on Android 6.0. The application we developed uses the device's main microphone to process the audio input. While the device was not configured to the ISO standards, the data can be considered a reasonable approximation of the noise levels to which SkyTrain riders are exposed.

What did we measure?

Below, we will analyze the intensity of sounds by measuring the Sound Pressure Level or SPL (expressed in decibel, dB) as recorded aboard SkyTrain cars with windows closed. The Sound Pressure Level is unweighted which means that it does not mimic the way the human ear perceives sounds through the A-weighting filter.

How will this study help?

The application we developed allows us to identify and log different sources of noise, calculate their intensity, and filter out specific sounds. In other words, we are able to filter out human sounds (i.e. chatter) and synthetic sounds (i.e. announcements, ringtones, music) to leave us with mechanical sounds. This study will help us analyze the noise levels commuters are exposed to during their ride and will also allow us to identify SkyTrain cars that require special attention, enabling the Operations team to make Skytrain rides quieter for the benefit of commuters and residents.

A Deep Look Inside The Data

We can find 3 different types of cars on the network. Mark 1 which are the oldest cars and set to be phased out by 2027 [iv], Mark 2 which were purchased between 2000 and 2010 and finally, the Mark 3, the most recent cars purchased between 2016 and 2020.[v]

Are Mark 1 really noisier than Mark 2 and Mark 3?

The table below is the output of the Tukey's Honestly Significant Difference (HSD) test. It is a way to find out which groups' averages differ from each other and which ones do not.

What it tells us is that there are statistically significant differences in the average decibel levels between Mark 1 and Mark 2, as well as between Mark 1 and Mark 3. However, there is no statistically significant difference between Mark 2 and Mark 3.
This means that the data we collected allows us to say that Mark 1 are indeed noisier than Mark 2 and Mark 1 are noisier than Mark 3. However, there is no clear evidence to suggest that Mark 2 cars are noisier than Mark 3 or vice versa.
By riding on a Mark 1 car, riders are, on average exposed to 82 dB levels while conversely, riding on a Mark 2 exposes riders to 78 dB and 79 dB on a Mark 3.

How loud is the SkyTrain fleet?

Rather than sampling the entire SkyTrain fleet, to assess how loud SkyTrain rides are, we can simply infer it using our sample. To obtain the intervals below we have used a statistical method called Bootstrapping which allows us to create artificially larger samples based on the original sample.

Assuming that our sample is representative of the entire fleet, we can say with 95% confidence that the average decibel level experienced while travelling on : - Mark 1 is between 82 dB and 84 dB, - Mark 2 is between 77 dB and 80 dB, - Mark 3 is between 78 dB and 80 dB. The more data we collect, the more accurate those intervals will be and can become one of the main Key Performance Indicators of the Noise Mitigation Program as recommended by SLR in section 7.0 of their 2021 report.

Which cars are the noisiest?

The correlation heatmap below shows the interrelationships between metrics such as the median, mean, amplitude or standard deviation change.

The average (mean) decibel level and the noisiest (max) decibel level recorded show a close positive correlation of 0.8 which means that we would expect the highest sound level exposure to take place in Mark 1. That is exactly what we see in the violin plot below. The highest level recorded on a Mark 1 was 103 dB, 98 dB on a Mark 2 and 95 dB on a Mark 3.

The table below lists the 10 noisiest cars surveyed sorted first by the mean decibel level and then by the highest decibel level recorded. Not surprisingly, out of the 10 noisiest cars, we find 8 Mark 1. This information can help prioritize which Mark 1 should be scheduled to be phased out.

The most concerning finding is that cars 446 and 463 which were acquired between 2018 and 2020 are already showing signs of extreme wear and exposing riders to sound levels as high as Skytrain cars that have been in service since the 1990s. This table can be updated regularly and become an indicator of which car needs to be prioritized for maintenance, upgrade, retrofit or replacement.

How noisy should we expect SkyTrain cars to be?

The data we collected can help us establish thresholds to define and compare a best case from a base case and a worst case scenario. The two dotted lines in the figure below show two important thresholds.

Any SkyTrain car with an average lower than 77 dB (25th percentile) would be considered a Best Case Scenario. Any SkyTrain car with an average higher than 83 dB (75th percentile) would be a Worst Case Scenario. Anything in between can be considered a Base Case Scenario.
A fully interactive scatter plot is available here. In the short term, the Mark I scoring the highest averages and highest decibel levels should be considered to be replaced in priority by the newer Mark V. Cars scoring between 81 and 82 dB should be a source of concern as they start to show signs of wear and age, and should be investigated to assess whether some maintenance or upgrades can be completed. Below are three illustrations for each scenario.

Best case scenario

The average noise level is below 77 dB and the highest recording does not exceed 85 dB as illustrated by a 22 minute ride on car 438.

Base case scenario

The average noise level is between 77 dB and 83 dB and the highest recording occasionally spikes over 85 dB as illustrated by a 24 minute ride on car 431.

Worst case scenario

The average noise level is greater than 83 dB and the highest recording regularly spikes over 85 dB as illustrated by a 23 minute ride on car 142.

Data Visualisation

Abdulsaheed Durodola, a Software Engineer and Developer based in Scarborough, ON, developed a web application to share the findings of this study in a way that could be implemented and used “to assist with ongoing assessments and business decisions related to the noise mitigation program.”
The dashboard allows user to interact with the dataset and to visually compare how rides on different car models varies. Below are two of the visuals available through the application. The heatmap below is a great tool to identify patterns, trends or variations within the data.

The scatterplot below shows the fluctuations of the Sound Pressure Level (SPL) while commuting on SkyTrain.

The people behind the numbers: A case study of Burnaby

In addition to the 43,400 people [vi] who commute daily on the section of the network we have studied, we estimate that approximately 69,700 individuals (28% of Burnaby's population) live in the 35,200 residences located within hearing distance of the SkyTrain tracks. According to Statistics Canada's 2021 survey, 3,055 of these residences were occupied by single-parent families and 11,535 were occupied by people living alone.

Burnaby hosts 11 above ground stations and between 18 and 19 kilometers of SkyTrain tracks go through North and South Burnaby. This accounts for 30% of the network’s tracks. That is more than Vancouver and Surrey combined. This snapshot emphasizes the significant impact noise mitigation efforts would have on both commuters and neighbouring residents in Burnaby and throughout Metro Vancouver.

Takeways

The data highlights the benefits of monitoring of sound levels inside cars. As Mark 1 cars are being phased out and replaced by Mark V, we showcase a plausible and realistic way to identify cars that need to be decommissioned in priority based on the level of noise they expose commuters to.
Another benefit of this approach is being able to identify Mark 2 and Mark 3 cars showing premature signs of wear. This would give the Operations team the opportunity to monitor which cars require maintenance and upgrades.
Eventually, a similar approach could be applied to cars on the Canada Line as many of them have been operating on the line for 15 years.
A proactive strategy led by data-driven insights is essential. The cost-effective approach we have presented allows the tracking of changes over time, anticipating wear and tear and ensuring the comfort of SkyTrain rides while maintaining that SkyTrain remains an attractive mode of transportation in Metro Vancouver and a good neighbour to people living near the SkyTrain tracks.

i. https://public.tableau.com/app/profile/translink/viz/2022TSPR-RailSummaries/1_SkyTrainandWCELineSummaries
ii. https://www.translink.ca/-/media/translink/documents/plans-and-projects/skytrain-noise-study/skytrain-noise-report-20181128.pdf
iii. https://www.translink.ca/-/media/translink/documents/plans-and-projects/skytrain-noise-study/skytrain-noise-study-phase-two-technical-report.pdf
iv. https://www.translink.ca/news/2023/august/first%20glimpse%20of%20new%20mark%20v%20skytrain%20cars
v. https://en.wikipedia.org/wiki/SkyTrain_(Vancouver)_rolling_stock
vi. https://public.tableau.com/app/profile/translink/viz/shared/C3SMHBRPN