mapArea field for Bing Traffic Events XML Data feed

I am trying to use the Bing Maps "Traffic Incident Data":
https://msdn.microsoft.com/en-us/library/hh441726.aspx
to feed a web application with traffic data. 

I cannot figure out how to dynamically determine the MapArea parameter.  This parameter is supposed to represent a bounding box defining an area by specifying SouthLatitude, WestLongitude, NorthLatitude, and EastLongitude values. (e.g. 45.219,-122.325,47.610,-122.107)

Ideally, I would want to pass my current longitude, latitude and radius/range (say 200 km) and get back a MapArea value of 37,-105,45,-94 for example.

The question is, how do I derive this bounding box area?  Is there a function or API to derive the MapArea ???

Any help and/or suggestion would be greatly appreciated.