Starting on July 8th, Transport for London (TfL) will start tracking passengers’ phones on the London Underground by default. Wi-Fi access points across 260 of the capital’s stations will track customers using their phones’ MAC addresses, which will allow TfL to see the routes they take through the network as well as through individual stations as they move from platform to platform.

Since the system relies on MAC addresses that phones automatically send to Wi-Fi access points when they try to connect, the only way to opt out of this system is to turn your phone’s Wi-Fi off entirely. However, as Wired notes, TfL will anonymize the data it collects. MAC addresses will be tokenized, meaning they will be replaced with an identifier that can’t be traced back to a smartphone or the customer who owns it. The transport authority says it will not collect any browsing or historical data from devices.

Its introduction follows a four-week trial conducted in 2016

The purpose of the tracking is to better understand how people use the Tube network and to provide better real-time information about crowding levels in stations. TfL can already use tickets to understand which stations people are traveling between, but starting later this year, London Underground staff will have access to crowd data to give people better advice on how to make their journeys. TfL also plans to issue crowding alerts via its website and social media channels. Perhaps more importantly, crowd data will also be provided via the authority’s existing APIs, theoretically allowing mapping companies like Google Maps and Citymapper to tailor their public transport recommendations to avoid areas of high congestion.

TfL says the scheme will also be used to benefit its advertising space by “highlighting the effectiveness and accountability of its advertising estate based on actual customer volumes.” If a high number of customers use a certain corridor in one of its stations, then TfL wants that data to give to potential advertisers.

TfL says it worked closely with the UK’s data privacy watchdog, the Information Commissioner’s Office (ICO), on the rollout of the scheme, and that its MAC-anonymizing solution has received the regulator’s full approval. Signs will be erected around stations, similar to existing CCTV warnings, to inform customers about the tracking program.

The introduction of the scheme follows a pilot that TfL conducted back in 2016. During a short four-week period, TfL says it collected 509 million individual data points from 5.6 million mobile devices traveling as part of 42 million journeys. Now, with its permanent scheme, TfL will quickly collect billions more. That’s a lot of responsibility, and the ICO hopefully had a lot of faith in TfL’s data privacy policies to allow it to happen.