Why couldn't they catch the early train?
They were delayed by______
答案是heavy traffic
为什么不是a heavy traffic
答：楼主的问题其实很有深度.
名词的可数与不可数,很多时候是相对的.有不少不可数名词,抽象名词也好,物质名词也好,在一定条件下可以转化为可数名词.比如：rain,a heavy rain,楼主印象中有“不可数名词前如有一个形容词就可以在前面加a"这样的概念,这是错误的,也是危险的.现在知道了,不是所有的.都能加a 的.这就对了.
查traffic的英语解释：
Vehicles or pedestrians in transit:
交通：流动中的车辆或行人：
heavy traffic on the turnpike; stopped oncoming traffic to let the children cross.
收费高速公路上交通繁忙；中断迎面而来的车流让孩子们过街
1 vehicles moving along a road or street 往来於街道的车辆; 交通
heavy/light traffic 往来车辆很多[不多]
There's usually a lot of traffic at this time of day.每天在这段时间往来车辆都很多.
Traffic was brought to a standstill by the accident.出了事故以後,交通陷於停顿.
London-bound traffic is being diverted via Slough.开往伦敦的车辆现绕道经过斯劳.
似乎不能加a,因为不是一次,不是一个,也不是一阵 的概念.这与a heavy rain不一样.
