Hi,
You can use the "Generate Near Table" function using the well points as the input and your roads as the near features. The output table will give you the distance from all the well points to the nearest road. From the table, you can simply summarize the distance field using the mean statistic. See the help for this tool below:
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//00080000001n000000
Caleb