Review and Critical Analysis of International UHI Studies

temperature decreased as water retention increased and evaporation efficiency deteriorated as
the material lost water. The experiment accuracy was about 10% compared to the weighing of
materials [229]. A complicated range of factors (apart from the albedo and emittance ‐ accounted
for to some degree in most studies) affect the temperatures of the pavements, thus more data on
all types of cool pavement technologies operating in a range of different climates and functional
conditions are required to determine the exact impacts pavement temperatures and thus on UHI
mitigation.
Seven studies examined the costs associated with constructing or implementing various cool
pavement technologies [027, 028, 068, 070, 073, 112, 187]. Few studies provided a detailed cost
breakdown for each individual technology ‐ reasoning for absence or generalizations can be
attributed to the costs of cool pavements being dependent on several complex factors: region,
local climate, contractor, time of year, project size, design life, etc. Most studies, however provide
a hypothetical breakdown or specific scenario analysis [070, 073, 068, 112]. One economic
estimate indicated that cooler pavements are more expensive by up to $10/square foot for non‐
vegetated permeable pavement blocks vs. conventional asphalt [073]. Most studies highlighted
that there are a range of benefits and drawbacks that need to be accounted for in any costing for
cool pavements (e.g. life time extensions of roads due to lower temperatures, reflective
pavements may imply lower night‐time artificial lighting, etc.). For example, simulations
conducted in one study showed that pavements that were cooler lasted 10 times longer than
hotter pavements, and pavements that were 22K cooler lasted up to 100 times longer [074]. There
is the need further investigation in order to understand the life‐time cost and the implications of
the cool pavement that are proposed to have a lifetime on average of 20 years [068, 112].
Studies indicate that the potential benefits of cool pavements in certain climates/conditions and
the drive to reduce urban temperatures has generated a growing interest predominantly in the
research aspects of the cool pavements rather than policy mainly due to the lack of data and gaps
in knowledge. However, in the US there is a drive in parallel to begin to create test procedures,
ratings and labels for cool materials such as cool pavements [028, 061, 074].
Three studies acknowledged the loss of reflectivity due to: weathering, accumulation of
dirt/pollution, use, age, etc [070, 071, 112, 128, 134]. One study tried to quantify the aging effects
on new conventional asphalt surfaces – demonstrated that for new surfaces having an albedo of
0.04 and after about 5 years having an albedo of 0.12 +/‐ 0.02 ode to a combination of: wear
(exposing lighter aggregates); oxidation; soiling/dirt of lighter color [128]. Another study
conducted by LBNL provided a comprehensive analysis of varying the albedo characteristics of
cements of varying composition – they found that weathering of cement can decrease albedo by
as much as 0.19 [134]. Further research is required in order to assess the relative
benefits/drawback this change in albedo may have on the performance of both cool and standard
materials.
Apart from one study in North America [021] the studies didn’t focus on the cool pavement
penalties in hot vs. cold conditions and similarly these conditions at different geographical
locations – all studies except one conducted in Greece were US centric. Little attention was given
the effects of increases in temperatures due to pavements or the effect of the sky view factor on
the effectiveness of any cool paving technologies. Moreover, factors such as efficiency and
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