Ada County Highway District

ACHD's Road Wizard Sunday, February 10, 2019 ACHD's Road Wizard

Lane lines hard to see in low-light rain and snow conditions; speed limit change is coming to Idaho 21 near Columbia Village

The Road Wizard Replies

Dear Road Wizard: As I have been driving around this week in the rain, I noticed that the lane lines are very hard to see. And at night if the road is wet or snowy, they are impossible to see.


Road Wizard:

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.

Dear Road Wizard: I am wondering why the speed limit is 55 mph on Idaho 21 near Columbia Village. It is difficult and dangerous to pull out onto the highway from the neighborhoods. It used to drop to 45 mph at the fire station; now it doesn't change until after Federal Way; going from 55 mph to 35 mph with no warning signs to slow down, so people blow through that intersection at 55 mph. I see aggressive driving and poor decisions every day. Why can't it be 45 mph after the bridge over the river?


Road Wizard:

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.

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