In the past few years,traffic problems(become)___more and mo

答案：have become
In the past few years 表示“在过去的几年里”,通常与现在完成时连用,而现在完成时的结构是：have/has + 过去分词.

In the past few years 意思是“在过去的几年里”，需要用现在完成时have become。
在过去的几年里，交通问题变得越来越严重。