Here in Massachusetts police can (in some cases) seize a driver’s keys during a traffic stop without the seizure amounting to an arrest.
The key case on this issue (no pun intended) is Commonwealth v. Moses, 408 Mass. 136, (1990) where the SJC wrote:
The pertinent inquiry is whether the degree of intrusion [i.e., key seizure] is reasonable in the circumstances. The degree of intrusiveness that is permitted is that which is “proportional to the degree of suspicion that prompted the intrusion.
In that case, a Boston police officer observed a car parked in the street. Men were standing alongside the vehicle. When they saw the officer, the men standing outside the car fled and one of the three men inside the car ducked out of sight. The officer approached the car and ordered the driver to hand over the key.
According to the SJC,
[the officer] was acting as a reasonably prudent police officer, and his actions were similar to and consistent with the protective measures…The taking of the keys in the circumstances was a reasonably prudent protective measure and did not change the investigative stop into an arrest.
To read the case in its entirety, click here.