This question is one of the easiest to give a response to and does not appear to carry any risks at all with the response. Quite obviously you do need to indicate that you are willing to travel as if this was not part of the job then presumably the employer would not have asked you the question. It is important not to imply that you are a free wandering spirit who tends to spend considerable amounts of time travelling but it is important to indicate flexibility and confirm a willingness to travel should the job require it. If you know the job is desk bound in one particular area it may be worth clarifying with the interviewer whether this is the case or whether they anticipate there being external travel involved.
Examples of Answers
“Yes, should the need arise I am more than happy to travel.”
“Yes. You can see from my CV that the former posts I have had travel has played a fairly important role and I am quite comfortable spending time visiting clients or sites.”
“I understand that this particular role is based in Chelmsford. I was not aware that travel would be part of it and wonder whether you could clarify this for me.”