Based on several calls to XM customer service about extending the subscription, I was told that the traffic information cannot be obtained without adding it on to one or another base XM radio service and that it is a $3.99/month add-on.
We have been quite impressed with the accuracy and detail of the traffic alerts on the Volt's XM trial period, so I had called to extend that subscription and with the arrival of the LEAF, I checked with XM about a "two car discount." Well, after hemming and hawing and considerable dickering, I was able to get them to give me basic XM radio coverage AND the traffic service on both cars for a 3 year extension of the trial for "only" $11.11 per car per month and billing will be 4 times over the 3 years.
Given that my wife will use one or the other car for her commute from W. Sacramento to UC, Davis about 12 miles to the west of our house and that there are a couple of ways to avoid major jams on the core freeway, the traffic alerts could easily save her both time and headache stress, so the subscription seemed easily worth that "discounted monthly" fee.
But my message to others here is that the posted XM monthly fees are "subject to at least some discount" if you call and show some interest but hesitate over the full cost....