It was a long and winding road to the June 1, 2019, opening of State Highway 45 S. Many residents of southern Travis County were eager to have commuter traffic diverted from neighborhood roads, but environmental groups raised concerns about damage to water quality and wildlife.

SH 45 SW has averaged about 9,000 cars a day, exceeding original expectations. The Central Texas Regional Mobility Authority is conducting an analysis of neighborhood roads such as Brodie Lane to determine the extent to which SH 45 SW is alleviating traffic there.

CTRMA Executive Director Mike Heiligenstein says the roadway provides a better experience for commuters and neighborhood drivers.

"The trip through the neighborhoods will be less," he says. "It’ll be a shorter time period also because there's going to be less people going through the neighborhoods."

For decades, environmental groups such as Save Our Springs Alliance maintained SH 45 SW would be worse for everyone because of potential damage to water quality and wildlife. Several lawsuits were filed over the years to try and stop the project.

Listen to KUT's interview with Heiligenstein to hear more about the project's potential environmental and traffic impacts:

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This transcript has been edited lightly for clarity:

Mike Heiligenstein: In this particular case we did an environmental document, but looked at the issues that were surrounding some of the concerns, particularly, where state regulations were saying one thing that we could have built this under, but we wanted to do a bit stronger. So we had quite a few stakeholder meetings that culminated in sort of a litmus test of things that we really wanted to see incorporated into this project. And they were elements that were applicable to this particular project because it is in the recharge zone. We realized that. So we really worked hard to see what level of environmental controls we could put on a roadway project that's 4 miles long that would exceed what was required.

KUT: The City of Austin had a State of the Environment Report that expressed some concerns about water quality – Edward's Aquifer, Flint Ridge cave and groundwater and nearby water sources. Can you talk a little bit about what the statement says about water quality?

Heiligenstein: Again, you're in the recharge zone and you're in an area that you have significant karst features. We are aware of that. We studied that upfront. We explored those situations, and then by the time we had a final design and entered into construction we totally – I believe – complied with the spirit and intent of the water quality regulations of the City of Austin.

KUT: Describe what a karst is, and how that feature played into the design and planning of SH 45 Southwest.

Heiligenstein: Throughout that part of Travis County you have a number of openings into the limestone formations, and a number of those outcroppings are cave openings very near the future roadway. But they're all over there. They're on farms or on ranches; they're in the neighborhoods that these cars currently use – Brodie, Manchaca - they're all over. So, you can't avoid them. You have to deal with them. You have to protect them.

We decided early on that we would build the roadway up as opposed to carving into Edwards. This protected the integrity of the limestone formation. But also we were able to drain water away from any of these karst openings in a much better fashion.

KUT: If there's a problem on those elevated roadways like a fuel spill, what does that mean for the surrounding area – the fact that it's up sort of instead of excavated or down?

Heiligenstein: Number one, then, you haven't scarred the surface of the limestone, so you haven't created any artificial openings. More importantly, in tandem with the fill, is the fact that we have hazardous traps on every culvert structure that's out there. We have a methodology for capturing a hazardous spill, which is over and above what a lot of roadway projects would do. And there's redundancy, meaning that not only are there traps, but there's vegetative swells or vegetative filter strips.

And then there are water quality ponds – several of those that would be a further opportunity; in fact, it would trap it. They’re lined just like a modern-day landfill. We really depended upon the science of how to keep anything like that from entering into the Edwards system, so to speak, the greater system of water flowing from one point to another.

KUT: What is the expectation for traffic habits and patterns changing?

Heiligenstein: What we think we're going to see – is what the modeling shows – is that people will give up their longer drives, their more congested drives, through those neighborhoods. People will shift over to 45, and you'll see less of an impact in those neighborhoods which would be really fantastic for them – that's what they're looking for. The trip through the neighborhoods will be less; it’ll be a shorter time period also because there's going to be less people going through the neighborhoods.

KUT: What some traffic folks will tell you is that when there are more lanes that means more cars because people hear, 'Oh, there's more space for me so now I'm gonna drive on this.’ When people hear, 'Oh, clearer neighborhood paths – now I’m going to go that way.' Is there any concern that they'll just be more cars overall?

Heiligenstein: Not in particular because actually the models take that into consideration - what the diversions are. And we know that a car on I-H 35 north of Kyle is going to stay on I-35. They're not going to cut over on this road. That's where you would pick up most of your other traffic would have been you know if I 35 was an impact.

But the numbers that I've seen – even if you opened it all the way to 35, which it doesn't do – it's a very small number of people that would divert. So I'm not particularly worried about that aspect. You've got to have more capacity. We have very little opportunity in the Austin region to expand that capacity, combined with travel behavior, which means travel demand management – getting people to do other things ­– I think all those are going to have to be wrapped up into a package that includes new roadway capacity.