Lerp creates a linear extrapolation between two given points; and, based on a percentage of the way between them, returns a value.
For example, if you have lerp(0,1,0.5) will return 0.5.
This is equivalent to using the linear equation: y =x
Because 0.5 is 50% of the way between 0 and 1.
Similarly, lerp(0,2,0.5) will return 1.
This is equivalent to using the linear equation: y = 2x
Because 1 is 50% of the way between 0 and 2.
Similarly, if you do lerp(0,1,0.3), you will get 0.3, and lerp(0,2,0.3) will return 0.6.
The lerp function does linear extrapolation for you, and thus would fill your role for a linear speed modifier well enough.