Show a map with a vector overlay using Leaflet
You can get more information from the blog at http://chris-osm.blogspot.co.uk/2012/06/leaflet-vectors.html
Download the code from github at https://github.com/chillly/speed-limits
This is an example of vector layers with Leaflet - DO NOT TRUST THE DATA, it is just an example and the data will be badly out of date.