Is it normal as an independent contractor to have different rates for different projects but for the same client?

I’m just getting started with my own environmental consulting business and I am applying for a contract currently. Some of my work may involve being out in the field and doing on-site consultation. It is my understanding that as a contractor I can’t change for travel time. If that is so, can I charge a higher hourly rate for work that requires me to make site visits at likely odd hours?