错在虚拟条件句的条件部分.提炼句子主干如下：The bus would not have had to take the long detour if the bridge didn't become treacherous.此句是对过去情况的假设：if从句谓语应用had done 形式；主句用would have done形式.所以从句应改成if the bridge hadn't become treacherous."如果（当时）那座桥不那么危险,公共车也不需绕道而行"
下面的链接有答案：