Last updated on May 22, 2012
A couple of days ago while driving on M-59 between M-53 and I-75 I noticed that the white dashed highway lines had been augmented with black rectangles on the distant (furthest away from you) side. I had a few theories about what these may be for, but the reality turned out to be a bit more mundane yet effective: additional marking to increase contrast on areas where sun glare is a problem, particularly on light-colored road surfaces.
This section of road was recently replaced with the new surface being all concrete, so it has a fairly light colored surface. When wet and/or with a bit of glare the markings practically disappear, as the embedded contraflective additives don’t really work, or reflect the same amount of light as the road surface.
I’d originally thought (and somewhat hoped) that these were augmenting existing markings to facilitate machine vision used by autonomous vehicles. Others suggested to me that they may be IR reflective to make markings more visible to night vision equipment. There was also the chance that it was a boondoggle to adjust the lane marker length to some standard.
This question was posed to Ask Metafilter and Facebook, email was sent to MDOT to ask, and I started doing some research. The most conclusive answer I could find was the augmentation of colored (white, in this case) markings to increase their visibility. Specifically, section 3A.05 of the Manual on Uniform Traffic Control Devices, 2009 Edition, Revision 2 states:
Black may be used in combination with the colors mentioned in the first sentence of Paragraph 1 where a light-colored pavement does not provide sufficient contrast with the markings.
and
When used in combination with other colors, black is not considered a marking color, but only a contrast-enhancing system for the markings.
I also eventually received a reply from MDOT stating:
The markings are part of a pilot project aimed at helping motorists see pavement markings during certain times of day when the suns glare/reflection is strong. The black outlines the white making the markings more prominent.
and
As I understand we will be putting more out on MDOT’s road in the next few years in locations where visibility by the sun is diminished.
So, that answers it. These markings are simply to increase contrast and improve visibility against the light colored concrete surface.
UPDATE: A photo of these lane markings can be see in this post.