are the illegal road patrolled by US police?
If its in the US and a widely known illegal crossing I would say it would attract both the US Municipal and Federal level police. I don't think it would be watched continually just a heavy patrol of CBP and the city police. This would, on the city side, be mostly a traffic concern and nothing to do with immigration. Their concern is the people standing around in groups on city streets are traffics hazards.
Note you WILL be arrested by the RCMP on the Canadian side for illegal border crossing. Expect that RCMP charge to make your relations with the Canadian Government a bit "rough" in the future.