- Full infiltration refers to a system where all water goes through Topmix to flow into the soil underneath. It's particularly useful in wet areas that don't need to collect the rainwater.
- Partial infiltration involves a semi-permeable barrier beneath Topmix that acts as a drainage system into nearby sewers or waterways — useful when the layer beneath Topmix can't pass the water through on its own.
- Full attenuation uses a capture system to store all the water that flows through Topmix. This option is most useful in areas with unclean water and high recycling rates, since the captured water can be reused later.
Cities can choose between the three systems depending on their needs, Tarmac says.
Topmix Permeable fares well in all climates except extreme cold, according to the company. In tests, it performed best in driving conditions where the speed limit was 30 miles per hour or less, and traffic was moderate to light.
Topmix is currently limited to sales in the UK. In its brief life span, it's been installed on a car park and a golf course.
Though the one-time cost of replacing infrastructural systems may be high — given that existing roads would need to be totally gutted — over time the reduced maintenance costs could make Topmix a worthwhile investment in countries prone to flooding.
Or those that simply want to be more like sponges.