What Can Cause a Brake Pedal to Vibrate at Highway Speeds?

My car’s brake pedal sometimes pulses when I slow down from highway speed. It’s much less noticeable during gentle braking around town, and I’m not sure whether that points to the brakes themselves or another part of the car.

For drivers who have dealt with this, what details helped narrow it down? I’m interested in what a mechanic would inspect when there’s brake pedal vibration, and whether it’s something that should change how soon I arrange a checkup.