Ana, you may want to invest in a Visa gift card once you get to the US. But even those have a $5 fee associated with buying the card. They are available in $50 or $100 increments and are as safe as travellers checks.
There are two issues, though. If you buy gas with them, they could put a huge authorization on it that could last for days before coming off (mitigated by prepaying at the window), and I don't think you get a PIN to use it as a debit card. I also don't know how they compensate for needing a zip code for verification, but I'm sure someone smarter than me has that figured out. Go check out visa.com.