A compound of the formula or a pharmaceutically acceptable salt, enantiomer, racemate, diasteromers or mixtures thereof, or a solvate thereof, formulations and methods of use thereof, as opioid receptor antagonists are disclosed wherein the variables are as described herein. A valve is operable to be placed in a first position to lead the whole flow of recirculating exhaust gases through the EGR cooler during situations where there is no risk of ice formation in the EGR cooler, and in a second position to lead the whole flow of recirculating exhaust gases through the bypass line during situations where there is risk of ice formation in the EGR cooler. When the pavement mat is compacted, the resulting ramp has an angle that is equal to or less than approximately 45\xb0 relative to a surface upon which the ramp is formed. The present invention also provides a system and method of screening test chemicals in fluorescent assays using photon reducing agents.