The IAPP has a good article on privacy considerations for things in cars and the data streams associated with them. Here’s a notable section:

On appeal, the California Appellate Court agreed, noting that “a person has no reasonable expectation of privacy in speed on a public highway” because speed is easily observable by the public, through radar detectors or estimation by a trained police expert. Thus, Diaz had no Fourth Amendment expectation of privacy; “technology merely captured information defendant knowingly exposed to the public.”

That doesn’t fairly reflect the actual court documents, which speak directly to the brake indicators being designed for public view.

…a person has no reasonable expectation of privacy in speed on a public highway because speed may readily be observed and measured through, for example, radar devices (e.g., People v. Singh (2001) 92 Cal.App.4th Supp. 13, 15 [112 Cal.Rptr.2d 74]), pacing the vehicle (e.g., People v. Lowe (2002) 105 Cal.App.4th Supp. 1, 5 [130 Cal.Rptr.2d 249]), or estimation by a trained expert (e.g., People v. Zunis (2005) 134 Cal.App.4th Supp. 1, 6 [36 Cal.Rptr.3d 489]). Similarly, a person has no reasonable expectation of privacy in use of a vehicle’s brakes because statutorily required brake lights (Veh. Code, § 24603) announce that use to the public.

The article makes many good points about secrecy of the logs collected, as well as manufacturer defects in devices that challenge the integrity of any observation (e.g. broken brake light). Above all, it’s interesting to see how courts ruled on something designed to publicly disclose, when the user claims its use was a protected secret.