This page shows you how to create a button that the user can click to get directions from their current location to a particular address using Geolocation.
This is intended for mobile devices, but will still work to a (somewhat inaccurate) extent in a good desktop browser.
If the user's browser does not support Geolocation, it will show the user a text input where they can enter their starting location manually. Try it in IE8 or below to see this effect.
Your browser does not support Geolocation. Please enter your postcode and click the button again.