Bystanders stood shocked on Elm Street last Monday as crowds of illegal dirt bikers and ATV riders tore down the road.

The riders on Elm are part of a long-standing problem for police in New Haven and its surrounding towns. The riders regularly gather in large groups and speed down streets, disrupting traffic as they pop wheelies and rev their engines of unauthorized vehicles not made for the road. Starting last spring, police in New Haven and in neighboring towns opened communication to track down illegal riders who block roads, violate safety laws and harass drivers and pedestrians.

“It was seemingly endless and packed with five or six guys [spanning the width of] the street,” said Kamil Sadik ’16 of the riders on Elm Street. “They were doing wheelies and it was kind of like a parade.”

These occurrences are not common on Yale’s campus, Yale Police Department Assistant Chief Michael Patten said in an email. While the YPD often collaborates with the New Haven Police Department on crime and safety issues — which include motor vehicle law enforcement — there have been no collaborations focused specifically on the dirt bike and ATV riders, he added.

While the streets within Yale’s campus are rarely filled with the roar of loud engines, these groups have been pouring into the streets of New Haven and its surrounding towns for over 10 years, said East Haven Police Department’s Lieutenant David Emerman. East Haven has experienced a particularly large number of riders over the past year, adding that the largest surge involved over 100 illegal riders — an unprecedented number.

According to Emerman, arresting individual riders is a slow and tedious process. The police often have to rely on videos posted by the riders on social media and tips from the public to identify riders, many of whom travel through neighboring towns, he said.

Because of the difficulty police face in arresting the individuals, police departments from New Haven, East Haven, West Haven, Branford, North Branford and Hamden have begun a cross-town collaboration to combat the riders. While each department only handles arrests in their own towns, the officers find it helpful to share their information, Emerman said.

Recent changes in the court system that allow police to confiscate the vehicles have also made it easier for police to prevent the illegal ATV riders and dirt bikers from filling the streets, Emerman said.

“We just want it to stop,” he said. “We don’t want to seize their vehicles but that seems to be the only thing that’s effective.” He added that, in recent arrests in East Haven, the police managed to take vehicles away from all riders.

The riders present a seasonal problem for commuters in the Elm City, said City Hall spokesperson Laurence Grotheer. Residents usually start noticing the riders as the snow clears in the beginning of spring and continue to be affected by their presence until the end of the warm weather months, he added.

Police have received reports from city residents that the riders are more than just a nuisance to drivers and pedestrians — they have also caused damage. The videos that the riders post online reveal that they have thrown rocks at cars and harassed drivers, Emerman said.

“It becomes a public safety hazard when these riders ignore traffic laws, including speed limits, traffic signals, stop signs and sidewalk use,” Grotheer said.

According to Emerman, while the problem has significantly worsened for towns like East Haven, the riders have had less of a presence in New Haven in recent years. He attributed the riders’ lower presence in New Haven to stricter policing methods, including putting more plainclothes police on the streets.

Police in seven greater New Haven towns have set up anonymous tip lines specifically for information about the dirt bike and ATV riders.