Ayutthaya police chief Sermkid Sitthichaikarn said young daredevils and their bikes were back to their old habits of disturbing the peace at several locations in the province from 11pm to 2am.
Pol Maj Gen Sermkid also conceded that fines and vehicle seizures did not scare the racers as they returned to their anttics again and again.
That latest race on Friday night turned fatal when a 20-year-old rider slammed into the rear of a truck on the outbound Asia Highway in Bang Pa-in district.
More than 100 motorcyles were using the highway to stage races and reckless driving led to the death, said Pol Col Chaiya Pethpanya, chief of Bang Pa-in police.
The victim was identified as Weerapat Pakka-at, a native of the district. The driver of the truck fled the scene, the officer said.
The military junta lifted the curfew nationwide on June 13 after it was enforced from midnight to 4am.
The National Council for Peace and Order has said a curfew could be reimposed in some areas if deemed necessary for security reasons.
The junta blanketed the country with curfews when it seized power on May 22 before easing the restrictions a few days later in some tourist areas.