Arizona state lawmaker apologizes for saying he has "immunity" from speeding violation

[SCantiGOP]

5. All states do this

Except for felonies or where a person has to be taken into custody to protect themselves or others, legislators are immune from arrest when traveling to and from the Legislature. They can, however, have the citation served once the session is adjourned.
This is an old holdover from the founding of the Republic when 13 independent nations were giving up their sovereignty to join together as the new nation. There were some who were afraid that, for instance, Massachusetts might arrest a New Hampshire congressman on the way to the capitol to prevent him from voting on some issue.

On edit: based on the post above mine, I guess some states have gotten rid of this archaic practice.