Forget the nonsense of needing to get at least 100 km past the border before stopping . That is a bit of " wisdom " someone started without too much thought.
If , as some sources suggest, the entirety of all the border states is under the sway of the narco cartels then why would any point beyond 100 km be any less risky than short of that ? This 100 km stuff is creating a false sense of fear or of false security.
If you go 100 km in from Piedras Negras you will be in some lonesome chaparral country but not in any town , having overshot Allende and falling short of Sabinas.
For the safety aspect it is only a serious concern if there may have been a flare up of narco vs police/army violence , so stay informed of those newsworthy events in the border points you are considering .

If all is calm then proceed with normal care and don't ride the open highways at night.

If you get to the border at Eagle Pass TX / Piedras Negras , Coahuila sometime in the afternoon with an hour or two of daylight left and with Mexico insurance looked after you do not need to drop anchor in Eagle Pass . The hotels may be relatively cheap by U.S. standards but that is deceptive.
Actually Piedras Negras is a much bigger city and they have a bunch of pretty good hotels where you can safely stay and pay less than in Texas.ive done it, so can you.
And it is quite safe to spend the night there, get some authentic Mexico food experience and head out at an early hour , after sunrise of course, and move on south.
Second alternative is to cross the Rio Grande , trundle through the border and through the city centre and head down as far as ALLENDE , Coah , it is only 50 km. Allende is the town here you need to arrange your Tourist Card and TVIP at the Mexican Customs Facility. You CAN NOT miss it as it still has the check lanes with roof spanning Mex 57 at the southwest corner of town.
If you just stick to 57 you will hit it. You can get your paperwork at any hour you choose.
Allende has about 5 hotels and they are all good and in a cluster on the north entry to town off Mex 57., There is hotel construction ongoing and a brand new one is super deluxe, but you can get a very good one for about $15 U.S. I know because I'm staying at one tonight, h&c shower, ac, TV , wifi and secure parking at my room door at Hotel Principal, across the street there is a nice restaurant , a Pemex and an Oxxo . All needs looked after in on stop.
This same scenario can be replayed at just about every border crossing from USA into Mexico , this year alone I've spent nights in Matamoros , Nvo Laredo and Mexicali. And I lived to tell about it .
The border cities are not the shooting galleries that some make them out to be . Sure if you go looking for trouble you can find it but behave well and stay out of the dark corners and booze halls and you will have no problems there

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http://advrider.com/index.php?thread....207964/page-5 then scroll down to post #93