Matthew J. Flannery
President And Chief Executive Officer at United Rentals
Thank you, operator and good morning, everyone. Thanks for joining our call. I'll start with the main takeaways from yesterday's release. In the second quarter, our team executed extremely well in a robust demand environment. And as a result, we delivered very strong performance by any measure. Our rental revenue increased by 26% year-over-year to a second quarter record of almost $2.5 billion, well above expectations and adjusted EBITDA grew faster than the top line, up 31% to a record $1.3 billion. We also demonstrated good cost discipline. Our adjusted EBITDA margin expanded 360 basis points to 47.3%. This contributed to a strong flow-through of about 65%. And importantly we delivered a 230-basis point improvement in return on invested capital to a record 11.5%.
The three tailwinds we saw at the start of the year continued to fuel our momentum. The macroenvironment remained favorable which created more demand in the quarter and you could see that in our rental revenue growth which included fleet productivity of better than 11%. In addition, the customer trend toward renting equipment is alive and well. We see this as a secular shift that will continue to move the market from owning equipment to renting it overtime. And lastly, we're confident that our growth is outpacing our industry as we continue to take share both in our core markets and with key customers. One reason, we're gaining share is our positioning as a one-stop shop. Customers place a lot of value on being productive. And our combination of scale, job site solutions, superior service and technology is unique in our industry.
Customers also care about safety and we prioritize safety on and off the job site. And this is another area where our team delivered in Q2 by keeping our recordable rate well below one. And increasingly customers place a value on sustainability. In May, we announced an initial agreement to purchase over 500 all-electric trucks and vans from Ford, including the F-150 Lightning pickups. Now this partnership is a good example of how we're continuing to add sustainable solutions to our rental fleet, while moving toward greener operations. We're proud of the progress we're making in many different areas of ESG including environmental stewardship and social impact. Yesterday, we released our 10th Annual Corporate Responsibility Report with comprehensive data covering 2021 along with more recent developments. You can find it on our website if you'd like to download it.
Another thing customers strongly care about is reliability. It's high on their list and we've got a very high bar in response. Our team is trained to deliver a caliber of service that earns the next opportunity. And our employees like that challenge and they love being an arrow to our customers. That's a big part of our culture at United and it helps with retention and recruitment. Our net headcount at the end of June was 9% higher than a year ago which is a solid gain in a tight labor market. Now I'll repeat something I said before, we're fortunate to have a world-class team standing behind our strategy. It gives us confidence in every target we put out there and that includes the updated guidance we released yesterday which raised our outlook for total revenue, adjusted EBITDA and free cash flow.
We have strong visibility through the balance of the year, and the activity we're seeing will create a lot of demand to get equipment on rent. There are plenty of positive signs to support this view. Virtually all of the external indicators are favorable including the Dodge Momentum Index, the ABI, contractor backlogs and customer sentiment. And the used equipment market remains robust. In the second quarter, we captured record recovery rates and margins on used sales. And I spoke to all of these dynamics coming out of Q1, and they're all still true today. Now I'm going to pivot to look at demand at the ground level. Our gen rent and specialty segments, both performed extremely well in the quarter. All of our regions company-wide, delivered double-digit rental revenue growth. And in many ways, it's a continuation of what we spoke about in Q1 broad-based activity across regions, stemming from a diversified mix of end markets and key verticals.
Looking at it by end market. Our rental revenue from non-res construction was up 27% year-over-year, and infrastructure was up 15%. And more broadly almost every vertical showed year-over-year growth in rental revenue. In terms of project types, large data centers are continuing to break ground along with infrastructure projects and distribution centers, and manufacturing is coming back. The power vertical is also accelerating and there are more tailwinds in the wings. With infrastructure for example, the funding is now finalized in Washington and we expect to start seeing a benefit in 2023 and beyond. With manufacturing, the resurgence of the industrial sector in North America, is being driven in part by supply chain challenges in other parts of the world and that's good for us. It's already evident in certain sectors.
Companies are investing hundreds of billions of dollars in mega projects in the US and Canada, to build plants across a variety of verticals like semiconductors and automotive. These projects will require equipment for years to come, and they play to our competitive advantage with large customers. On the specialty side, the segment had another excellent quarter led by our power and mobile storage businesses. The segment as a whole grew rental revenue by 39%, including the benefit from General Finance. Pro forma specialty was up a strong 29%. We opened 24 cold storage through June, in specialty against a revised target of about 45 openings by year-end and that's slightly higher than our original projection of 40 openings this year.
So as you can see, 2022 continues to be a landmark year for our company both financially and operationally. We delivered another record quarter in, what we expect to be a record year. Our flow-through in the quarter reflects the team's discipline in navigating a challenging cost environment. And we continue to have the benefit of a strong balance sheet, low leverage and robust cash generation. This gives us the flexibility to act opportunistically on many fronts. This year we expect to make the largest investment in our history in fleet of about $3 billion and our suppliers are taking good care of us, and our capex spend is tracking to plan. We'll also continue to explore growth through cold starts and acquisitions. We've made seven bolt-on acquisitions this year to date, for a total consideration of over $300 million.
Lastly, we expect to complete our share repurchase authorization this quarter. These are all prudent capital allocations, to create long-term shareholder value. And we know that the key to leveraging capital, is relentless execution and that's what you're seeing from us in our results. Now before, I hand it over to Jessica, I'd like to take this opportunity to thank her personally for her many contributions over the past seven years. As you all know, Jess will be leaving us to take on a new opportunity and Ted has stepped in as we go through the CFO search process. And I know I speak for our entire leadership team, when I say it's been a pleasure to work with Jess and we wish her all the best in her new endeavor.
And now with that Jess, you've got floor.