Don't use an expression.

1) Make a plane object with the UV grids exactly where you want your particles.

2) Select your plane and execute the "emit from object" feature.

3) set your emitter to be directional and set the rate to something low like 1.

4) Make sure to set the direction attribute to be whatever direction is right for your lane object and set the spread to 0.

You should have a nice grid of particles now.