Without altering colors, Cokin neutral density filters will allow you to gain the 1, 2 or 3 apertures you need to vary the background, distance and depth of field.
Neutral density filters allow you to use the low speeds needed to give an impression of speed.
Cokin's Neutral density filters are available in three grades and can be use in combination with almost and other type of filter.
Neutral Density 152 reduces the light 1 full stop.
Neutral Density 153 reduces the light 2 full stops.
Neutral Density 154 reduces the light 3 full stops.