How do I simplify this line graph -- over 200 line items??

A chart that has too much information on it can be hard to read. IMO, before "simplifying" a chart, you first need to consider exactly what you want the chart to show. A few observations to perhaps spur some thought:

1) I notice that the traffic at Litchfield is quite a bit larger than every other location. Is the comparison to Litchfield important, or would it be enough to state that Litchfield is significantly larger than the others and remove it from the graph?

2) Your data span about 2 weeks with several points per day. The data show both the intra-day variance and the day to day variance. Which is more important to see -- intra-day or day to day changes? If intra-day changes are more important, then choose one or two days that are most interesting and plot only the data for that day or two and ignore the other days. If day to day changes are more important, summarize each day into one datum (perhaps a daily average) and plot that one value per day.

3) Is it necessary to have all 18 locations on the same chart, or would you accept "representative locations" (Litchfield has the highest traffic, Sutherland is representative of the "medium traffic" locations, and Forbes Craig is representative of the "low traffic" locations) so that you only need 3 to 6 locations on the chart?

Those are the kind of questions I would look at. Consider those and anything else that you know is important to your desired goal, then we can figure out what the most useful way will be to simplify this chart.