📖 The Quick Answer
The forward camera identifies signs at the side of the road and matches them against a stored library of sign shapes. The currently active speed limit shows in the cluster. Many cars combine TSR with map data from the navigation system so the displayed limit is correct even when a sign is missed.
⚙ How It Works (Sensors and Algorithm)
A camera behind the rearview mirror runs image recognition on each frame, picking out the round (Europe) or rectangular (US) speed-limit signs by shape and color. The recognized number is cross-referenced with map data from the nav system. The result is shown as an icon in the instrument cluster or head-up display. On vehicles with Intelligent Speed Assist (ISA), the system can also chime when you exceed the limit, or even cap throttle.
🛡 What It Protects Against
Drivers from inadvertent speeding, missed signs in construction zones, and unfamiliar roads. It does not prevent collisions directly but reduces the likelihood of speed-related crashes and tickets.
⚠ Limitations and When It Fails
TSR can be confused by signs on trailers, signs visible on side roads, and electronic signs at unusual angles. Faded or graffiti-covered signs are sometimes missed. The system does not enforce limits, it only displays them (except on cars with mandatory ISA).
🚗 Which Vehicles Have It
Standard or optional on most 2019+ European and Asian brands, and on a growing number of US brands. Names include BMW Speed Limit Info, Mercedes Traffic Sign Assist, Audi Traffic Sign Recognition, Volvo Road Sign Information, Honda Traffic Sign Recognition (on 2022+), Toyota Road Sign Assist (RSA), Ford Adaptive Cruise Control with Traffic Sign Recognition, Tesla Speed Limit Sign.
🔧 Related TSBs and Recalls
Several TSBs address Tesla phantom speed limit changes (the car reading a limit on a trailer or service road). Most are software updates rather than hardware recalls.