关于“traffic”与be动词的搭配，需根据其词性和使用语境来选择：
作为名词时
- 单数主语：
- 现在时： is
- 过去时： was/were
- 示例：
- The traffic in the city is very heavy today.
- Yesterday, there was a major accident on the highway.
- 复数主语：
- 现在时： are
- 过去时： were
- 示例：
- The traffic lights are malfunctioning.
- Last night, there were several breakdowns on the road.
作为动词（及物/不及物）时
- 及物动词（如"traffic away"）：
- 现在时： traffic+ 宾语
- 过去时： trafficked
- 示例：
- The company's profits were traffic away by the new policy.
- 不及物动词（如"traffic lights"）：
- 现在时： traffic
- 过去时： trafficked
- 示例：
- The traffic in the downtown area is always busy.
补充说明
若需强调"交通流量"这一整体概念，常用 there is/are结构：
现在时：
- There is heavy traffic on the main road.
过去时：
- There were jams due to the snowstorm.
注意避免混淆：
不可数名词（如"traffic"）与可数名词复数在be动词选择上无本质区别，完全取决于主语是单数还是复数。