Matthew J. Flannery
President and Chief Executive Officer at United Rentals
Thank you, operator, and good morning, everyone. Thanks for joining our call. I know there's a lot of interest in 2022, but before I go there, I want to take a look back because the foundation for our current outlook can be found in our 2021 performance. I'll start with our strong finish to the year. As you saw yesterday, our record results for the quarter solidly outperformed expectations for growth and profitability. We grew fourth quarter rental revenue by 25% year over year and total revenue by over 21%, and our adjusted EBITDA was 26% higher than a year ago with a margin improvement of 170 basis points. This translated to a solid flow through of 55%. These gains capped a full-year performance that was far better than we could have imagined back in January our team is firing on all cylinders with strong execution in the field, solid cost control, effective investment in the business, and thoughtful management of our resources, starting with our talent base. In short, it's our people who outperformed expectations and our numbers reflected that.
I want to stay with this theme for a minute and summarize some of the accomplishments for the year. We maintained a strong safety record, finishing with a full year recordable rate of 0.79 and that was while integrating multiple acquisitions. We also grew our net headcount by 12%. Roughly half of that came through M&A. And the $1.4 billion of capital we allocated to acquisitions is generating attractive returns. In fact, our 2021 return on invested capital improved by 140 basis points to end the year at 10.3.
We also generated $1.5 billion of free cash flow last year after investing a record $3 billion of rental capex, and we sourced that equipment in the midst of a supply chain disruption. On the ESG front, our company recently earned an upgrade to an A rating by MSCI. We've received similar scores from other ESG rating agencies, reflecting our commitment to our progressive culture. Environmental, Social, and Governance matters have been drivers of value in our business for more than a decade, and it's gratifying to see that recognized.
Now on to 2022. As you can tell from our guidance, we're very confident in our industry's outlook for strong growth this year. A number of key indicators have been moving the needle higher for months, including the broad recovery in construction and industrial demand, the continued strength of the used equipment market, and an economy that's moving in the right direction despite some lingering challenges. Given these dynamics, it's not surprising that industry sources show a steady increase in confidence among contractors, and our own customer confidence index improved throughout 2021, ending at its highest point at the end of the year. And importantly, the same optimism was echoed by our field leaders last month as we work through our annual planning process. And we heard it again at our virtual meeting. We had our annual management [Phonetic] meeting virtually two weeks ago, and this meeting is always a great opportunity to get everyone aligned on goals and strategies. And it's clear that our people are fired up for the opportunity. They can see the benefits of the countless improvements that we've made over the past decade, both operationally and also with our customer service and they know those efficiencies count for a lot as we grow the top line.
The biggest signpost pointing to ongoing growth in 2022 is the diversity of the demand that we're seeing in our end markets. In the fourth quarter, we grew rental revenue by double digits across all of our regions and all verticals showed positive growth as well, and these were solid increases with rental revenues from non-res construction verticals up 24% year-over-year and infrastructure up 11%. Industrial also grew 11% with strong gains in refining, metals and minerals, and power. And it's notable that both non-res and industrial picked up steam in the back half of '21 with year-over-year rental revenue gains in Q4 coming in higher than those of Q3.
Our specialty segment had another strong performance with every line of business growing double-digit year-over-year. The segment as a whole reported a rental revenue gain of 45%, including a pro forma growth of 28%. This year we're planning for around 40 cold starts in specialty following the 30 that we opened this last year. Specialty is key to our competitive differentiation. And given the segment's history of high returns, expansion will continue to be a priority for us.
I'm sharing these numbers to underscore the point I made at the start of my comments that the building blocks for our current outlook were laid in 2021. Our core markets have recovered faster than expected and the underlying construction and industrial forecasts are positive. The broad-based acceleration over the last 12 months has become the foundation for a new cycle of growth. And for the first time since COVID arrived, we're seeing a sustained improvement in long-term visibility, which gives us some insight into future market conditions. That's a huge plus for us after two years of uncharted waters.
I'll mention a couple tailwinds on our radar. One, of course, is the infrastructure bill, which will add an additional $550 billion of funding for projects directly in our wheelhouse over the next five years. We've been expanding our infrastructure capabilities for years. We have a rock-solid value proposition with traction in the right articles for this bill. We expect to see some benefits as early as 2023. Another tailwind in our future is the relocation of manufacturing operations back to the U.S. Onshoring initially drives demand for construction, followed by the need for our industrial services once they're up and running. The pandemic has caused manufacturers to rethink how they operate, and we've already seen some funding for new projects tied to this trend.
Along with the increase in customer demand, comes a large responsibility to have equipment available for rent. And I mentioned that we brought in $3 billion of fleet last year when equipment wasn't easy to find, and that was a home run for the company and for our customers. We're continuing to work with our strategic partners to land a similar amount of fleet this year.
And finally, before Jess goes over the numbers, I want to mention an announcement we made yesterday and a milestone that's coming up later this year. The announcement is our share repurchase program. We expect this program to return $1 billion to shareholders in 2022. And the milestone I mentioned is our anniversary. United Rentals will turn 25 years old this year, and as you know, we've been a growth story from day one. Even so, I don't think there's been a time in our history when our strategy, culture, and financial strength have been more of an advantage than they are right now in this new cycle.
We have a highly-engaged team, a cohesive customer service network, and industry-leading scale that matches the market opportunity. We've built these levers into the business to create shareholder value, and they did their job in 2021. Now we'll take that to the next level this year and for the foreseeable future.
And with that, I'll ask Jess to cover the results, and then we'll go to Q&A. So Jess, over to you.
