When traveling for business and you will be a passenger, it is possible to do work. Just keep in mind some considerations for other passengers and the driver.
If you have not been asked to answer the driver's cell phone or be the navigator you have the opportunity to work. You can use this time to compose emails, work on presenatations or other documents and use your cell phone.
Just remember that you must not provide distractions for the driver of the vehicle, so if you are making or receiving phone calls keep the volume and your temperment down.
Also keep in mind that you should not discuss nor disclose information that others may not have access to or the need to know about.
It can be difficult when traveling in a car with others not to let them be aware of what you are doing. Use discretion and if you realize that there are too many distractions, put your work away. Attempting to work with too many distractions will make you frustrated and increase the odds you will make errors.