Michael Schoch, Answers questions on Noodle
Since you already have your bachelor's it sounds like you'll be applying to a graduate program. Here is a link to a list of the top civil engineering programs in the U.S.
Many of the top programs, such as University of California Berkeley have prerequisites for graduate and doctoral candidates that can be viewed on the engineering department's webpage. For transportation engineering, students are expected to apply to the program with at least a 3.5 GPA, which is roughly a 90% average. It may be pretty difficult to gain admission to such a school with a GPA that's much below the minium.
That said, other factors beyond your GPA are considered when applying to graduate school. Work or internship experience, or academic awards in your field may boost the competitiveness of your application. Similarly, it's always advantageous to have high GRE scores. You may want to consider investing in some test prep classes or tutors to make sure you achieve the highest score possible.
A good strategy is to make a list of programs and call or email each of them directly. Ask about the program's prerequisites and what you can do to supplement your school transcripts.
You should check out some of the articles and advice available on Noodle's topic pages for ideas on how to strengthen your application.
I hope this is helpful. If you have any more questions feel free to ask a follow up. Best of luck!