Students and professionals from a variety of domains have demonstrated different approaches to problem solving. These two populations have displayed differences when using and perceiving multiple representations of problem-solving tools. In the domain of transportation engineering, this difference has yet to be evaluated in detail. This study addresses that knowledge gap. We used a mixed-methods approach with measurements of eye movements for visual attention and reflective interviews to gather participants' reported representation use and problem-solving assumptions with three taxonomies of representations (equations, graphs, and flowchart) because they solved an open-ended design problem. Visual attention (VA) was recorded with a head-mounted eye-tracking device. Reflective interviews were used as a self-reported depiction of overt VA on the selected representation, and to record assumptions made by each participant to solve the problem. Equations, graphs, and flowcharts received different magnitudes of statistically significant VA for both groups. Professionals had a significant difference in VA between the flowchart/equation and graph representations, whereas students showed a difference in all three categories. Professionals generally chose representations with higher complexity (use of different combinations of representations) than students, as reflected by their frequency of conjoining representation choices and their associated assumptions. Professionals commonly approached problem solving by documenting specific assumptions, whereas students approached the problem more generically. Efficient information extraction occurred for professionals, but it took more time for them to solve the problem than novices. Novices frequently utilized the flowchart. Differences in performance may be explained by professionals using conceptual frameworks, or rules for interpreting and navigating the problems that were potentially developed through exposure to the domain of knowledge and deep understanding of the subject matter.