Why don't you believe the reason that my car was held up in the traffic jam. 

句意：你为什么不相信我的汽车被交通拥挤阻塞了这个理由呢?

分析：that 引导的是定语从句。这里的  that 相当于关系副词 why 或 for which，其实也可以省略不用。即：

Why don't you believe the reason that my car was held up in the traffic jam. 

=Why don't you believe the reason why my car was held up in the traffic jam. 

=Why don't you believe the reason for which my car was held up in the traffic jam. 

=Why don't you believe the reason my car was held up in the traffic jam.