Well the first thing you need is a feature class or shapefile of the interstates. Won't get far without that.Once you have that, do you need just the total length, or do you need mile markers? For mile markers I think you need M-values in the interstate data. To just get total length, clip the interstates by the Los Angeles County feature, right click the Shape.length field (or another distance field, if present), click Statistics and the Sum value is your total length in whatever units the distance is in. Then divide by whatever number is appropriate to get miles. For example, if the distance is in feet, divide by 5280 to get miles. If you need mileage for individual freeways, select them first.You may want to add a field to interstates and calculate mileage from your distance field directly there. Then the Statistics -> Sum on that field would be your answer.
Here is what I tried to do. I tried to make a new layer of the LA county the includes CA interstate within it. My problem is I can't completely contain all the interstate within the L.A. country.
Do not select the interstates. Use them with the Intersect Tool. Put both the interstates and the County boundary in the same layer with LA County selected only and this will cut the lines at the borders. Then Dissolve the lines as necessary by the Highway number (if you need miles by facility) and get your lengths.
Richard,Wouldn't Intersect also result in inclusion of interstate segments that have segments crossing the county border, as well as any outside the border but touching it? That's why I suggested Clip.
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