yep, echo would probably be my first choice for something like this, mainly because you can choose an operator like add or maximum which will help intensify the lights. it also has control over intensity decay, which may be helpful.
you may also want to try cc wide time. it will work similar to echo, but it uses frames in front and in back of the sampled frame (echo only work in one direction).
enabling frame blending by itself may help for sped up footage (layer>frame blending>frame mix). ae will then composite the frames that are dropped out when you speed up footage. it will also help in conjunction with echo and cc wide time (with cc wide time you also need to set the effect to use ‘native motion blur’ to take advantage of frame blending).
even after all that you may still need a little directional blur or radial blur (depending on the movement of the cars) to help smooth things out. glow might add to the effect…
Kevin Camp
Designer – KCPQ, KMYQ & KRCW