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    What's the difference?

About 10 years ago, Orange County, . Fla., began to pave its dirt roads with porous asphalt, expecting them to last only a few years. Instead, most are still in use, saving the county money, eliminating equipment, and reducing maintenance.

Orange County is one of the fastest-growing urban areas in central Florida. More than 2500 lane miles—ranging from seldom-traveled rural byways to unpaved enclaves in urban areas—serve the county's 1 million citizens. Until 1997, more than 220 of these miles were high-maintenance dirt roads that literally bogged down traffic during rainy season and kept a fleet of seven motor graders constantly moving during dry season.

The challenge for the Orange County Roads and Drainage Division, which maintains the roads, was to find an affordable way to pave them.

Gravel roads require a lot of maintenance, which the division was trying to eliminate. Conventional paving was the ideal solution, but too expensive; it would cost $250,000 to $500,000 per lane mile, depending on right of way and drainage issues. In 1997, the county could afford to pave only 3 to 5 miles per year; and at that rate, it would take more than 45 years to complete the project. In addition, conventional paving would raise major stormwater permitting issues. Several roads didn't have enough right of way to construct conveyance systems, and the division couldn't afford designing and permitting the systems anyway.

Porous asphalt, however, circumvented all these issues. Porous pavement allows water to seep through the pavement and into the ground, eliminating the need for stormwater conveyances. And it was affordable: It would require less permitting and less raw material than conventional paving (to see a cost comparison between conventional paving and porous pavement, click here.

The division initiated infiltration and permeability tests to verify that the pervious pavement runoff coefficient was equal to or less than that of the existing dirt roads. The two local water management districts that required this information, St. Johns River Water Management District and South Florida Water Management District, would approve all permits as long as the pavement wouldn't be seal-coated and roads wouldn't be re-aligned.

In addition, the division had to:

  • Minimize wetland impacts (separate dredge-and-fill permits are required when proposed roadwork will negatively affect wetlands).
  • Leave existing drainage systems as-is. In addition, peak discharge rate for the affected areas could not be significantly increased.
  • Excavate detention swales on both sides of the pavement (if possible) to provide temporary storage of water during rainy season.
  • Connect the detention swales to the outfall system using pipe culverts or overflow weirs.
  • Agree to not add lanes to existing roadways.