Should I wait to go graduate school for policy or engineering?

Congratulations on almost being done with your engineering degree!  I am not familiar with how to utilize dual degrees between public policy and engineering.  However, I don’t think you have to consider a completely separate degree just so you can write in your engineering career.  In my company, our engineers (including myself) do a great deal of writing especially on our planning, traffic and transportation analyses, research, and engineering opinion type of projects.  We write reports for Municipalities, DOTs, developers, neighborhood associations, etc.  Our company has approximately 25 engineers and only about 5 of us are great writers.  We even had to do training with our engineers to enhance our technical writing skills.  The struggle we find is that engineers often have a hard time separating themselves from the technical side of projects enough to be able to adequately describe in writing the project to non-engineers.  A lot of engineers will even tell you they became engineers because they were good at math, not writing.  I can assure you that being an engineer that is capable of clear writing that effectively communicates your engineering project to a non-engineer, you will set yourself apart from your competition.  When you start looking for jobs, consider ones that have a planning, research, or analyses component side to it as these would be the most likely areas to have writing in your daily tasks.  Another thought to consider is that in order to fully utilize your engineering degree, you have to gain four years of experience under a Professional Engineer (PE) in order to test for your PE license.  Going into a separate non-engineering career (such as public policy) will interfere with a timely completion of your PE track.