Highway driving is typically far more fuel efficient than driving in the city. Constantly accelerating and decelerating causes the vehicle’s engine consume more gas. The same idea happens in the body while exercising!
The body loves to run at the same pace, or, on cruise control. Increasing intensity, even for just 10-15 seconds, shakes up the energy systems that supply our muscles with fuel. A big difference between humans and vehicle is our bodies adapt to the stress we inflict, automobiles do not!
To perform an acceleration, increase intensity from the current pace to an all out sprint. Once finished, slow back down to the previous constant pace. Start with 4 x 15s with at least 3-5 minutes of recovery between each one.
Adding accelerations is a great way to get some added speed work during a recovery day.