In December 2021, the City of Tucson laid down its first cool pavement as a pilot. The product used - PlusTi from Pavement Technologies - includes titanium dioxide to reflect UV rays back into the atmosphere. It is basically a mineral sunscreen for the road.

The City chose this product because it is visually indistinguishable from normal pavements. The product goes on as a yellow-beige with the titanium dioxide suspended within an asphalt rejuvenator. Within an hour or two, it has soaked in, doing its work for as long as the pavement exists. No need to restripe. Also, given the sun and heat exposure in the region, we can take whatever rejuvenation product we can get on our roads!

In anticipation of this pilot, our team sought funding from the National Institute of Transportation and Communities to (a) do a pre/post evaluation of heat and (b) consider the technology in the larger context of multi-modal travel.

The evaluation of this cool pavement pilot is exploring both a pre/post and case/control design to document changes in surface temperatures, ambient temperatures, and thermal comfort. We anticipate lessons from this evaluation will be highly applicable elsewhere as cities go all-in on cool pavements as part of the environmental-justice oriented $500 million Healthy Streets Program passed in Congress in 2021.

Cool pavements are well tested in laboratory settings, but less studied in real-world settings. This is partly due to the newness of the technologies. But it is also related to the difficulty of controlling for street types, green space along the street, and weather.

Addressing variation in weather can be somewhat controlled through careful consideration of data collection timelines. We went out on 3 days in late October to gather "pre" data. October was chosen due to funding cycles and the need to not have high humidity levels that occur only in July-September. We will go out in early March to match approximate day length and high temp patterns. Within models, we will control for background ambient temperatures. We generally collect data during the heat of the day (10am-3pm) for maximum surface temps and at dawn to understand the potential for overnight cool-off.

The City of Tucson, in coordination with the vendor, chose the pilot site - a 1.5-mile length of Country Club south of Broadway and north of Aviation Hwy. The research team noted three distinct street typologies within the chosen site and is playing with a methodology to "match" the sites with nearby controls that account for the width of the street, green infrastructure, and presence/absence of pedestrian facilities (sidewalks).

IR (Heat Gun) measurements were taken on the hour (starting at 10:00 AM) with a repeat measurement approach. All measurements taken with the heat gun were logged on REDCap to secure data measured. Measurements were collected at each site on the nearest road, sidewalk, and gravel or grass surface.

In addition to basic information such as collector's name, segment, and location, we logged:

1) Surface composition

2) Whether the surface was in the sun, shade, or partial shade

3) Surface Temperature readings (repeated at each point 5 times) from the heat gun.

Measuring ambient air temperatures, even at a microscale level, does not give information about how a person will feel about the heat in the context of all weather conditions such as moisture, winds, or evaporative cooling. In Arizona, it does not capture the vast swing in comfort between sun exposure and shade. It also does not capture the amount of heat radiating up from surfaces such as the road or nearby concrete block walls. This is why using a thermal comfort index is critical when discussing pedestrian, cyclist, and transit rider comfort along our roadways.

Wet Bulb Globe Temperature (WBGT) is one of the major thermal comfort or personal heat exposure measurements used throughout the world. It expands the concept of ambient air temperature to incorporate solar heat that beats down on the Kestrel 5400’s black, copper globe and humidity measured at the Wet Bulb. Wind, ambient air temperature, humidity, and solar radiant heat are all considered in order to approximate thermal comfort. WBGT is the standard for occupational uses such as for the US military and athletic outdoor sports.

The Kestrel 5400 provided us with WBGT measurements every 10 seconds from 10 A.M to 4 P.M and were placed as mentioned in the previous section.