I bench 120kg+ without a spotter (at 95kg body weight). If I were to fail disastrously, I might have a broken rib, but without collars on the bar the chance of a severe failure that cause me to drop the fully loaded bar on my chest is pretty much 0. I've put it down on my chest and rolled it off more than once, and it only causes a light bruise that's gone in a few hours. I probably wouldn't do much more than 1.5 times body weight without a spotter, but I'm assuming you're not there yet, as I suspect you wouldn't be asking those questions if you were.

And use the dumbbells. You can bench with them. Do sumo deadlifts (legs flared much wider than usual, and lift the weight between your legs - you can start with one dumbbell, and increase to two). Learn to clean the dumbbells for overhead presses and front squats. Back squats are tricky with dumbbells, so there you'll have to resort to the Smith machine, thouhg it is considered somewhat evil amongst many lifters (it is easy to end up lifting with bad form and an unnatural movement, and very easy to fool yourself by leaning into the bar), but it is better than nothing.

Frankly, until you start getting to dumbbells that add up to more than your body weight on every main exercise, there's not much reason why the lack of a proper bar should impede you, and dumbbell exercises have much to be said for them for stability.

You can bench without a spotter pretty safely up to about 75kg, at which point the roll of shame starts getting challenging. It's different for women though, because rolling a large amount of weight over breasts is apparently quite painful.

Benching with dumbbells is fine, and potentially a better workout than a barbell bench anyways.

Leave the collars off the bar. If you get into trouble, dip the bar to one side and drop the weights.

Note: I haven't tried this with a huge amount of weight, so it might still be dangerous if you're benching 300 lbs+

Consider getting the Free Spotter (http://www.shermworks.com). Its about the safest way to go, and is endorsed by a few professional lifters. Nothing in your way to corrupt the motion, and you simply let go if you get in trouble. You don't even need a rack for bench or shoulder press, as you can set the empty bar at your chosen height, load the weight, and lift!

To be frank: you switch gyms.

You don't need to bother benching without a spotter, if you feel uncomfortable. You can load those muscles with body weight exercises. (Push up variations, dips, handstand push ups, etc.)

Alternatively, benching dumbbells won't leave you trapped below a barbell, even without a spotter.

Benching without spotting is easy if you aren't going to one rep maxes. If you are doing sets of 5, then don't do rep N if rep N-1 was too hard that you think you will fail the rep. Also, if you stop before the 5th rep, you should be doing sets at a lighter weight.

You might want to try out low weights but lots of reps. See my reply above about trying out low weights, high reps to the point of exhaustion.

If your gym only has a smith machine it certainly isn't dangerous to bench; dumbells also offer many exercises, you can even bench with dumbells. Needless to say though, if that's all your gym has, change gyms.