Twitter! Wow a new source for constituent concerns. But not all answers are 140 characters long. Usually I speak to people by phone or e-mail (almost never get a written letter) but since the issue of an advance green arrow at Laurelwood and Erbsville is public on twitter, I will answer on my blog. Ah, the modern world!
A regualr reader found staff’s reply confusing. Oh that engineer language! I will try to make it clear by bolding relevant points.
We are talking about what is called a left turn arrow where a green arrow on the traffic light allows cars to turn left while the opposing traffic and traffic on the left and right still have a red and are stopped. Otherwise, cars are unable to turn left.
Here is staff’s reply:
Jane,
I will have staff look into the advanced left-turn at Laurelwood. It should be noted that our research suggests that additional left-turn phasing does not improve safety at intersections and can actually increase the number of collisions. Therefore we do not be consider this suggestion to be a collision countermeasure. Adding a left-turn phase also takes time away from the pedestrian walk interval; a disadvantage to the pedestrian. Alternatively, fully protected left-turn phasing (when left turning motorists are permitted to turn left on an exclusive green arrow only) is considered a collision countermeasure (reduces accidents), however this proven collision countermeasure has greater impacts to delay for all users of an intersection.
Additionally, when you add a left-turn phase, it takes time away from the opposing traffic, as well as other movements, thus resulting in fewer gaps in opposing traffic for the remaining left-turning motorists. More importantly, our most common type of pedestrian collision involves left-turn vehicles searching for gaps in opposing traffic, committing to their turn and striking pedestrians leaving the curb.
For these reasons, we only install protected/permissive left-turn phasing when it is required for capacity / demand reasons.
Jane again. I have contacted Waterloo Police to check out the kids crossing against the light.
Jane Mitchell's Blog 
A quick note – opposing (southbound) traffic is minimal; the majority of SJAM’s catchment area, and the related traffic come from south of the school. There was a letter to the editor in the Record blaming tie-ups on Erbsville on the roundabout (false, in my opinion). The traffic jams are north of the roundabout, and likely because of the signalized intersections. Thanks for looking into this.