Our quiet roads will be jammed with cars coming from other towns, especially on Saturday morning.

jammed 是动词 jam 的过去分词形式，jam 本意是"塞满、堵塞"，指道路或空间被车辆或人群挤得水泄不通的状态。这里用被动语态表示道路"被车辆堵塞"的情况。