Adjust the time offset and the number of echoes. If objects are moving very fast across the screen then they may move too far between frames to get a smooth streak. It all depends on the motion.
It the test footage I'm using its not moving all that fast, and I have my time at 0.009, and number of echo is 70. I have tried all sorts of numbers for the time as low as .00003 all the way up to large numbers. But still no streaking. Any idea of what number may produce streaking?
A combination of "warp time" and "edge smooth" might be able to get you what you're after. "Motion 2" (a multi-script preset tool) by Mt. Mograph uses this method to generate liquid-like morphing between motion animations, though i'm not sure if it only works on a layers bounds or if the same combo could be used to displace actual pixel values like you need. Another suggestion would be one of the vector based warping tools offered by Re:Vision.
By the way - i believe the look you're after is more commonly referred to as "ghosting" in the video world.
You can calculate the time offset by measuring how far the light source moves between frames and factoring the duration of a frame. If the footage is 29.97 fps then each frame equals 1/29.97 or 0.0333667 seconds. If you want to echo each frame right behind the other then that would be the echo time. It's the default for the effect. IOW, if the comp is 29.97 fps then each echo is one frame behind the previous one. If you shorten that time and you are using progressive footage then you are going to get duplicate frames. If you increase the time then you will get more than one frame between each echo. It works that simply.
If you have an animation that you pre-compose and then apply echo AE can and will calculate the in-between positions of the animated elements and give you smooth footage. If you are using progressive footage then AE will not be able to calculate the position of the pixels between the frames without adding things like time remapping and frame blending to the mix. So here's the deal, say you have a taillight and it's 10 pixels wide, but it moves 20 pixels per frame then no matter what the echo time there's going to be a hole between the echoes. If the taillight moves 10 pixels per frame then you'll get a bunch of connected dots. If it moves 5 pixels per frame then you'll get the overlap that will produce a streak. Here's a circle that moves exactly 10 pixels per frame but it's is 40 pixels in diameter so there is overlap:
In your second sample in the first post it looks like you got streaks from the blue lights. In the first example either the circle is moving too fast or you have too much time between echoes.
I hope this helps.