Well, they're slowing it down anyway.
I'm not sure if they are really going to allow Mexican trucks to go from a US shipper to a US destination, though. I think this is simply to allow them to pick up a load in Mexico and deliver it in the US, then pick up a load in the US and deliver it to Mexico. Currently, they have to stay within 20 miles of the border and then the load has to be unloaded and transferred to an American truck.
Heck, it might even be good for us. We'll be able to take loads to Mexican destinations too. Currently, for instance, my company sends a few trucks a week to Brownsville, TX where the load is transferred to a Mexican truck.
Basically, this would simply be allowing open access both ways, just like we currently have with Canada. I agree with the safety issues, though... Some of their trucks are just scary. In fact, their best trucks are our throwaways.