ACHD's Road Wizard Sunday, February 10, 2019

Visible lane lines are something road agencies want to provide, but the weather conditions where drivers benefit most from them are also what makes them difficult to see.

Lane paint has retroreflective glass beads embedded within it, but rain can form puddles over the markings. In low-light conditions, the water can get in the way of headlight beams reflecting the paint back to drivers. And winter weather conditions can cause paint wear because maintenance vehicles are coming and going, treating the streets with chemicals and armed with plows. Plus there is the normal wear and tear from motor vehicles.

ACHD has installed raised pavement markers in some locations, which help in the rain especially, but those tend to get popped off by snowplows. ACHD has also experimented with recessed pavement markers, but they tend to get covered with dirt and grime.

So, refreshing existing paint is usually the best approach, which ACHD does each spring and summer, as weather allows. This year a "high build" paint will be used for the first time that should result in thicker, more durable lane markings. The additional thickness should help the stripes stand out better in wet conditions, and hold the reflective glass beads a little better as the paint wears.

The way speed limits are set can seem counterintuitive because it is based more on how fast people are driving rather than how fast agencies or residents think people should be driving.

Transportation agencies start with the "85th Percentile" rule when determining speed limits. It is the speed 85 percent of motorists travel at or below on a given stretch of road. The theory behind this method, supported by data, is that most people will drive at a speed that is reasonable, regardless of the posted speed limit. If the speed limit is set lower than what most drivers feel is "comfortable," some people will obey the limit no matter how uncomfortable they are, while others will drive what feels right regardless of what is posted. This creates speed differentials that can be less safe compared to similar streets where drivers are all traveling more closely to the same speed.

The Idaho Transportation Department manages Idaho 21 and evaluates the highway for speed limit conditions every five years. A study just wrapped up and found that a change from 55 mph to 45 mph is more appropriate between Technology Way and Federal Way.

The limit won't be dropped after the bridge. It will stay at 55 mph until Technology Way, because most people are comfortable driving that speed.