I played with this topic a lot some time ago. You can find some examples here:
 Dobrou extensions plugin examples
 Dobrou extensions plugin homepage

Using Sporttracks and this plugin, gps tracks can be converted to KML and displayed in Google Earth. It supports some ideas mentioned here - track coloring based on color gradient, direction arrows and much more.

Some examples:
 3D wall - combination of more things is used here. 1) Arrows showing direction, with actual speed as label. 2) Color of every point choosed from gradient depending on speed. 3) 3D wall - elevation of track "wall" above terrain based on speed.
 
When using only one color for whole track, it's easier to compare speed between more tracks.
 
 Shadow effect - number of tracks going same way is visualized here, but can be customized to visualize speed, etc.
 
 Arrow race - size and color of arrows are based on speed.
 

I played with this topic a lot some time ago. You can find some examples here: 

Dobrou extensions plugin examples 

Dobrou extensions plugin homepage

Using Sporttracks and this plugin, gps tracks can be converted to KML and displayed in Google Earth. It supports some ideas mentioned here - track coloring based on color gradient, direction arrows and much more.

Some examples: 

3D wall - combination of more things is used here. 1) Arrows showing direction, with actual speed as label. 2) Color of every point choosed from gradient depending on speed. 3) 3D wall - elevation of track "wall" above terrain based on speed. 

When using only one color for whole track, it's easier to compare speed between more tracks. 

Shadow effect - number of tracks going same way is visualized here, but can be customized to visualize speed, etc. 

Arrow race - size and color of arrows are based on speed. 

I played with this topic a lot some time ago. You can find some examples here:
Dobrou extensions plugin examples

Using Sporttracks and this plugin, gps tracks can be converted to KML and displayed in Google Earth. It supports some ideas mentioned here - track coloring based on color gradient, direction arrows and much more.

Unfortunately, because of some spam prevention, I can not send more links or images. So please follow provided link to see examples...

I played with this topic a lot some time ago. You can find some examples here:
Dobrou extensions plugin examples
Dobrou extensions plugin homepage

Using Sporttracks and this plugin, gps tracks can be converted to KML and displayed in Google Earth. It supports some ideas mentioned here - track coloring based on color gradient, direction arrows and much more.

Some examples:
3D wall - combination of more things is used here. 1) Arrows showing direction, with actual speed as label. 2) Color of every point choosed from gradient depending on speed. 3) 3D wall - elevation of track "wall" above terrain based on speed.

When using only one color for whole track, it's easier to compare speed between more tracks.

Shadow effect - number of tracks going same way is visualized here, but can be customized to visualize speed, etc.

Arrow race - size and color of arrows are based on speed.