Global Mapper GIS Tool
Global Mapper is a popular and low cost GIS tool that can be used (like most GIS tools) to draw DSRC MAP-SPaT center-lines on base maps such as Google or Bing and then export those lines in formats like KML/XML or GeoJSON for further use.
You can download a copy at: http://www.bluemarblegeo.com/products/global-mapper.php
It is not free, but it’s cost is is very modest when compared to many other GIS tools. They maintain an excellent user forum board at: https://www.globalmapperforum.com/
Here at SCSC we have used this tool for DSRC Map creation and many other GIS needs for well over a decade with good results and great support. We most often use it in our precise servery automotive GNSS work to produce base maps for testing differential corrections needs. It is one of the few GIS tools we trust at a millimeter levels of precision. Like most “real” GIS tools, it requires a certain investment in effort to learn. Most of the non-Google based map shown on this site are licensed to SCSC by way of this product.
FYI: There is a rich API available for this tool that makes it a candidate you may want to consider if your goal is to create a MAP-SPaT specific editor to meet DSRC market needs. The only similar alternative would be the open source project QGIS.