While I understand everyone’s desire to drive supplies into Houston, I can tell you from my experiences during Katrina that truck loads of random donations create a huge logistical mess. Sorting through your used goods to determine their usefulness and get them to the right place takes a whole infrastructure itself.
Your BEST WAY to help is by donating money to TRUSTED organizations that are already on the ground doing the work – they know exactly what supplies they need and simply need the resources to purchase them. I recommend HARVEY DISASTER RELIEF because 100% of donations go to relief efforts. The cooperative giving of churches cover the entire administrative and marketing costs of the organization and your donation can be COMPLETELY focussed on relief efforts.
I know it FEELS better to load up the car with supplies and start driving but giving to a trusted organization will actually accomplish exponentially more.