Converting color raster to vector is difficult. For black and white raster, it's easy. I have not seen a single converter that satisfactory converted color raster to vector. Even the touted "Vector Magic" has issues. The best kind of .svg's are drawn not converted. The most optimal ones are done not using any proggy such as Illustrator or even Inkscape. I've look at the original speedwarn_list.svg with a header comment of
<!-- Generator: Adobe Illustrator 16.0.2, SVG Export Plug-In . SVG Version: 6.00 Build 0) -->
which mean it was created from Illustrator and it's not optimized. Tons of id's used without definition of it, so it's not relevant and can be removed.