Final answer:
The transition from double time to quick time is accomplished in one step when the command 'Quick Time, MARCH' is given. Double time is a faster marching pace, while quick time is normal pace. Soldiers 'route step' across bridges to avoid forced oscillations and resonance.
Step-by-step explanation:
To transition from double time to quick time, a command is given during the march for the transition. Typically, when the command 'Quick Time, MARCH' is called out, the next foot to hit the ground marches at quick time, which is the normal marching tempo. The transition happens in one step after the command is executed. Marching at double time means the troops are moving at a faster pace, specifically at 180 steps per minute. In contrast, quick time is a regular marching pace at 120 steps per minute. This transition ensures synchronization and maintains the uniform movement of a military unit.
In regards to soldiers being instructed to 'route step,' or walk out of step, while crossing a bridge, this practice is because of the phenomenon known as forced oscillations and resonance. Military commanders order soldiers to break step to prevent a rhythmic step that could potentially resonate with the natural frequency of the bridge, potentially causing structural failure due to the amplification of vibrations.