The US Customs and Border Protection site gives information on your husband's type of situation. They answer the question "Will I be arrested when I re-enter the U.S. if I have unpaid traffic tickets?"
Customs and Border Protection (CBP) consumer center answer is that they strongly advise you to pay your traffic tickets, particularly moving violations. While unpaid tickets would not subject you to arrest unless a warrant is issued, you may be subject to a more intensive inspection if your record is not clear.
So it does not appear that your husband would be denied entry or arrested at the border however he may be put under more scrutiny by the CBP. If your husband drives in Vermont and is stopped again by law enforcement then it is likely he would be arrested. Normally if you do not pay your traffic tickets then you are convicted in court for failure to pay or failure to appear on the citation and an arrest warrant is issued.
Another question asked to the CBP is "Can I be admitted into the U.S. if I have either a misdemeanor or criminal record?" Customs answered that:
The U.S. does not deny entry to persons with a "Driving Under the Influence" conviction - although if there are multiple convictions for this and or other misdemeanors, you could be denied entry. Generally, any convictions for drug possession can result in denial of entry. If the conviction was long ago, you might contact the U.S. Embassy, Office of Consular Affairs in your country to obtain a waiver. Other misdemeanors may result in denial if they were recent.
The CBP goes on to say there are many reasons for a person to be denied entry into the U.S. The most obvious are if a person has previously worked illegally in the U.S., is suspected of being an intended immigrant (i.e. planning on staying in the U.S. past the terms of their admission), or of having ties to terrorist or criminal organizations. But there are many more reasons for persons to be excluded from the U.S. Among them are having been found guilty of crimes of moral turpitude (Child molestation, rape, fraud, theft, etc.), having been found guilty of a criminal offense (for instance murder or grand theft), having overstayed a previous visit to the U.S. or if the visitor is suspected of having an infectious disease. In addition, tourist visitors to the U.S. are expected to have sufficient funds to support themselves while here - in order to ensure that they will not attempt to find employment in the U.S., which is a violation of the terms of the tourist visa.
Since the speeding ticket in VT does not appear to fall under any of these categories it sounds as if your husband will be okay to get into the U.S. however to make certain that he does not run into problems either at the border with Customs or with law enforcement inside of the United States he should look into taking care of the unpaid traffic ticket. If the Vermont citation is dealt with then you can go on vacation to Boston without worrying about the VT unpaid ticket becoming an issue and tainting your trip.
If he does not remember where in Vermont he received the ticket he can try contacting the VT Department of Motor Vehicles. If he does know where the ticket was received then the county court should be able to advise him how to now take care of the ticket and clear his record.
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