You cannot always ask permission, of course. If you are shooting a street scene or a wide shot of a market, you can't run up to everyone and ask if it's OK. In general, people do not mind this sort of photography—it's only when they're singled out that they get uncomfortable. But not always. Be sensitive to the scene in your viewfinder. If people are getting nervous, ask permission or move on.
Travel photography as a genre is one of the most open in terms of the subjects it covers. Many travel photographers specialise in a particular aspect of photography such as travel portraits, landscape or documentary photography as well as shooting all aspects of travel. Much of today's Travel Photography style is derived from early work in Magazines such as National Geographic magazine from photographers such as Steve McCurry. This genre of photography entails shooting a wide variety of subjects under varied available conditions, e.g. low light photography indoors, available ambient light photography for exteriors of buildings and monuments, shooting on the streets where sometimes conditions may be hostile, capturing moments which rarely recur, capturing the magic of light while shooting landscapes, etc.
The more willing you are to travel at a moment's notice, the more opportunities you can access. Day jobs will limit travel, so will mortgages and car payments. Photojournalist Lynsey Addario recently wrote about being 7 months pregnant while on assignment in Gaza. I deeply admire her bravery and commitment to her work, but I imagine many photographers aren't willing to make such compromises. Consider your lifestyle, and how much time you're willing to spend away from home. As for myself, I'm a long-term digital nomad traveling with a suitcase and a backpack and an open mind. Being available and flexible has made a monumental difference to my career.
Besides the travel publications like National Geographic Traveler, Conde Nast Traveler, etc., the demand for this genre exists in industries like Travel, Photo Education, etc. Many travel photographers are today leading photo-tours through companies such as Intrepid Exposures, utilising their knowledge of unique travel locations, experience of working as professional photographers and using this to help travel enthusiasts take great travel images during their trips. Many others are doubling up as educators in the field of ambient light photography. Some of them are doing assignments which intrinsically use their strengths, e.g. shooting exteriors or interiors of buildings for architects and interior designers. Photographers like Steve McCurry are often commissioned to shoot commercial advertising work using their skills from travel and documentary photography to produce powerful advertising images.
Aga Szydlik is a professional culture photographer and a doctoral candidate based in South Africa. She tells us that her journey with photography started with Muay Thai (the famous Thai fight style) which she documented extensively. Based in Thailand, she able to explore South East Asia, onwards to Indonesia and South Africa. She is enthusiastic about alternative processes, analogue photography, Lomography and salt/albumin prints as well as mixed media.
I don’t do a lot of big stock photography, but I do stock images with TandemStock and they have done quite well for me. I haven’t been stocking with them for long, but I do leave a lot of my adventure and outdoors style photos in their hands. I earn about $150 a month with them for now, which doesn’t sound like a lot. However, they have made some high-profiles sales for me. For example, I was the background for Bing. I love TandemStock, and I hope they start to grow a bit.