Can a digital SLR camera take long exposures for "traffic streaks" or "fireworks"?

yep, no problem and you don't have to deal with the reciprocacy of film.

sure can. just set your mode dial to shutter priority and set it for as long as you can in seconds.

It can as long as you use Manual mode. You'll need a tripod and a cable release (unless your camera allows you to set long exposures). If using a cable release it has to be the old fashioned mechanical kind. An "electric" cable release will not hold the shutter open. Once contact is made the shutter closes.

For "Traffic Streaks"

Find a good vantage point away from extraneous lights. Set your camera to Manual, focus to infinity, the f-stop to f8, the ISO to 200 and try a 15 second exposure. (This works equally well for distant city skylights). If the scene appears too light for your tastes try a 10 second exposure.

I once set up my tripod in the front seat of my car, keeping the lens as close to the windshield as possible. Using a 28mm lens at f8 I drove around (at night) making 15 second exposures using a long cable release. My camera has a motor drive so advancing the film was automatic.

I have also photographed a city skyline (f8, ISO 200, 15 second exposure) and zoomed from 70mm to 210mm during the exposure. This has to be done very smoothly so as to not disturb the camera.

For more information on shooting fireworks, visit fotosharp.com and buy their 'Day & Night Exposure Guide'. It is an invaluable tool

As a short and sweet answer, like other posters have said, yes, a dSLR can take long exposures for the subjects you've mentioned. However, most cameras today can take longer exposures, it just depends on your definition of "long" (seconds? minutes?) as well.

Are you shopping for a camera exclusively for the ability to take long exposures? If you are, then I would advise against you buying a digital SLR for the reason I mentioned- basically any digital camera from the most basic point and shoots to the most advanced SLR can take long exposures, it all lies in your ultimate motivation or reason to buy the camera. Are you looking for specific features, such as the ability to change lenses to take in different perspectives of fireworks or traffic? If you need that feature, then a dSLR would be for you. Do you get my drift? Basically, if you are looking for specific features, then say so so that the answers may be better tailored to make better recommendations.

You don't need a dslr for that. A lot of compacts these days like my canon A640 have a shutter speed as low as 15 seconds and that's probably enough for what you want.

YES

With the use of a tripod (or table top tripod) it's as easy as 1,2,3 ... manual or shutter priority. Just use a long exposure and f/8, f/11 or f/16, plus take some notes. A table top tripod works great for vacations, on top of a bench, trash can or car, just be creative.

you dont need any more than a few seconds. fireworks, ferris wheels, cars at night..they all do really well on 1-4 seconds.