I've actually driven up to a very large (more than 1 foot across) snapping turtle.  While it may not be the answer you want, you simply don't.

Things I've tried:  This is based on actual experience.

 - Lift them from behind:   While easy to run around behind them, it doesn't work.  Little known fact, they can "leap" very small distances, easily out of your hands. 
 - Lift them by the tail:  Risk of spinal injury for the turtle, and dangerous for you unless you have rather long arms.
 - Flip them over:  No good.  I tried prying with a 3" diameter branch... can't do it.  They have *very* strong legs and it's really hard to flip them.

I gave up when it snapped and broke the 3" diameter branch in half.  Noting that my tibia is less than 3" diameter I decided to leave nature to its thing.

The best you can hope for is to block the road until they finish their journey.  Unfortunately they are likely to just chill and not move while you are close by.

As Sue reiterates in her comment, the most likely outcome of trying to help a snapping turtle is that you injure/maim the turtle.  Close after that is the likelihood that you suffer serious injury as snapping turtles can easily remove parts of your body.

Even the official Wildlife Division approves: 

> Snapping turtles should never be picked up by their tails as this can
> damage the animal's vertebral column and tail, not to mention the
> human who is in danger of being bitten. Because snapping turtles can
> be slimy and heavy, the Wildlife Division does not recommend that
> anyone manually pick them up.

From [Connecticut's Official State Website][1]


  [1]: http://www.ct.gov/deep/cwp/view.asp?a=2723&q=469200