Unless you have a Victorian ladies style frame, crossing your leg in front of you is a fine way to crash.
bicycles.stackexchange.com/users/2358/cmannett85
While I strongly recommend coming to a complete stop , I would think that standing on your pedal and extending the other leg out behind you to cross over would be safer.
In the cross-in-front alternative, your pedal foot is stuck behind the leg that hits the ground
and I would expect there to be a significant risk of getting tangled up while in this 'grapevine' position and tripping.
bicycles.stackexchange.com/users/213/Gary.Ray
I agree with previous answers, cross the leg behind you.
But, if there is a reason for which you prefer to do it crossing the leg in front of you, in the final dismount movement, I recommend to hop out of the saddle and land with both feet on the ground more or less at the same time but with one in front of the other (Preferably the one that crossed from the other side should be in front).
This maneuver should allow you to quickly star the run or walk prior to the complete stop.
bicycles.stackexchange.com/users/2177/Jahaziel
Moving the leg around behind you is better because after it's around, you can put it on the ground without crossing your legs.
In other words, suppose you keep your left foot in the pedal.
If you bring your right foot over the front, then put it on the ground, with your left foot still in the pedal and the bike still moving slowly, it tends to twist your body around to the left
so you're facing backward.
If you bring your right foot over the back, it's in a natural position for you to start running alongside the bike in motion.
Also, it's easier to bring your leg over the back because you don't have to bend your knee so much.
bicycles.stackexchange.com/users/2102/xpda
It really depends on what type of riding you are doing and whether or not you have clipless pedals.
If you are a cyclocross racer, the left foot remains in the pedal, and the right foot swings over the back wheel, then threads between the left foot and the frame to step on the ground.
This fast, but tricky dismount is explained in this video from a former cyclocross national champion.
Some triathletes use a similar technique.
If you do not have clipless pedals or your left foot is resting on top of your shoes, you can swing the right foot over the back wheel, position just behind the left foot, and then hop of the bike to land on your right foot.
The safest thing to do, however, is to stop and dismount.
bicycles.stackexchange.com/users/1953/Jeff Bloom
When doing a rolling dismount, one foot stays on its pedal, the other has to cross over somehow. Would you suggest crossing over in front of you (while sitting, folding you knee to allow the cross-over) or behind you (standing on your pedal and extending the other leg out behind you to cross over.  Note: I don't mean to know which you prefer, rather which is safer/easier/quicker than the other, and why.
Rolling dismount: leg crosses over in front of you or behind?
bicycles.stackexchange.com
<technique>
bicycles.stackexchange.com/questions/5979
bicycles.stackexchange.com/users/2360/Shawn
For a rolling dismount, unless you have a ladies bike without a bar, the leg must be crossed behind or your leg will hit the bar and you will either fall over or crash. Moreover, it is much safer to stop and dismount correctly. While different types of bike and riding styles can change the preferred moving dismount, bringing your right leg over the back and landing just behing the left is usually the most natural way to transition to running, and avoids tripping or twisting on landing. It is safer to stop first though.
Crossing in front is more dangerous unless you have a step-through frame. Crossing behind reduces the likelihood of tripping or twisting on landing. If you bring your right leg over the back and behind the left foot, it is in a good position for you to start running beside the bike. It is safer to come to a stop first. While different types of bike and riding styles can change the preferred moving dismount, bringing your right leg behind the bike and is usually the most natural.