City streets are usually paved with asphalt concrete because this material gives good service and is relatively inexpensive to construct and maintain. We show that making asphalt pavements cooler, by increasing their reflection of sunlight, may lead to longer lifetime ofthe pavement, lower initial costs of the asphalt binder, and savings on street lighting and signs. Excessive glare due to the whiter surface is not likely to be a problem.

In an earlier report (Pomerantz & Akbari 1998), the air-conditioning energy and smog reductions that might result from the use of whiter paving materials were estimated. In this paper we consider some of the collateral effects ofwhiter pavements — on their durability and on illumination they produce. Asphalt pavements are the most common street pavements in cities because they are relatively inexpensive to construct and maintain, and they give acceptable service. Such pavements are dark colored because a dark binder (asphalt) is coated onto stony aggregate in order to glue the aggregate into a rigid paving material. A newly constructed asphalt concrete’ (AC) pavement thus has the color ofthe asphalt, which is quite black. The color of the pavement has several important environmental consequences. AC contributes to sunlight’s heating of the air near the surface(Pomerantz et al. 2000). The dark color means that sunlight is not being reflected; the absorbed energy raises the temperature of the pavement and thus the temperature ofthe air that is near it. This immediately contributes to the heating ofthe city. When the temperature gets high enough, the modern response is to turn on an air conditioner that further heats the outside air and costs energy. The atmosphere also responds by using the thermal energy to drive the conversion of organic gases and nitrous oxides into smog. There is thus a cost in both energy consumed and degradation of the environment (Rosenfeld et al. 1998). These costs may be lessened by coating the pavement with a light-colored material (Pomerantz et al. 1997). We suggested in the earlier report (Pomerantz & Akbari 1998), in addition to detrimental environmental effects, the heating of pavements may be bad for the pavements themselves. The properties of asphalt binder are known to be temperature dependent. For example, the stiffness of asphalt decreases exponentially with temperature (Yang 1972). Likewise, the related property, viscosity, as measured by penetration of a sharp needle, decreases exponentially with temperature (Hunter 1994). The effects of pavement— temperature on performance have been recognized by the Strategic Highway Research Program (SHRP) which grades asphalt according to the pavement temperature range it will endure (Cominsky et a!. 1994). However, asphalt binders that function over wide temperature ranges are more costly. This opens possibilities for additional savings by constructing cooler pavements: by reducing the maximum pavement temperature, a lower grade of asphalt may be acceptable, and/or some failure will be delayed. Ultimately the lifecycle costs of maintenance and disposal ofpavements will be reduced. This paperwill investigate these non-energy or non-environmentally related effects of cooler pavements, both the potential benefits and detriments. Such benefits, in addition to longer lifetime, may include better visibility. The danger of glare seems to be negligible for the suggested reflectivity. We also report some measurements on the relationship between the reflectivity of aggregates and chip seals made of them. The evidence suggests that cooler pavements may offer impressive benefits to society, and thus warrant further study.

AC pavements fail by a variety of mechanisms, some of which are temperature dependent. Some failures might be delayed or eliminated if the pavements were more reflective of sunlight and their temperatures were thereby decreased. First we establish the order of magnitude of the effect of the reflectivity of a pavement on its temperature. The reflectivity averaged over the solar spectrum is the albedo2, a. Measurements (Pomerantz et a!. 2000b) were made of pavement temperatures in Berkeley and San Ramon, CA. In Berkeley, data were taken at about 3 PM on new, old, and light-color coated asphalt pavements. The data from San Ramon were taken at about 3 PM on four asphalt concrete and one Portland cement concrete (a = 0.35) pavements. In both places, the solar energy fluxes were about 1000 Wm2. A decrease of about 4 °C(7 °F)was observed for an increase of albedo of 0.1. (Fig. 1) (A change in albedeo of 0.25 is the difference between fresh black AC, with a = 0.05, and Portland cement concrete, with a 0.3.)

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