With the economy appearing to show some improvement – a recent report showed GDP grew at a 4.1% rate in the second quarter, the fastest in four years – and the nation's infrastructure in great need of an upgrade, now might be a good time for investors interested in the companies that might fuel any rebuilding.

There's no question the nation's infrastructure needs a reboot. The American Society of Civil Engineers, which issues a report card on U.S. infrastructure every four years, has given the nation a sad D+ in its two most recent reports, 2013 and 2017. Clearly, the need for an overhaul is there. But what exactly that means, who pays for it and how much should be spent is up for question.

In February, President Trump unveiled his administration's proposal: a new $1.5 trillion infrastructure plan that includes a $200 billion contribution from Washington over the next decade meant to spur another $1.3 trillion in spending by cities, states and the private sector. While it is unclear whether the specific plan will get through Congress – Democrats largely see the plan as falling short and Republicans are largely unwilling to push through another big spending measure – it is apparent that the administration is prioritizing rebuilding infrastructure, as has been expected.

The new focus on infrastructure is encouraging to investors who seek exposure to this niche, and Trump's infrastructure bonanza, if passed, could be a boon to big investors.

It is important to note that infrastructure is an extremely broad category, encompassing transportation, communications, water and electricity – representing hundreds, if not thousands, of equities. Selection is essential to success, since not all "infrastructure" stocks will take off under the proposed spending. (For related insight, read more about building your portfolio with infrastructure investments.)

That being said, well-diversified companies positioned to handle major infrastructure projects should do well over the medium- to long-term. Here's a look at four of the most promising infrastructure stocks. All figures are accurate as of September 30, 2018.

1. AECOM (ACM)

AECOM's business is designing, financing, building and operating infrastructure assets for federal and state governments and businesses. With a market cap of $5.455 billion and fiscal 2017 annual revenue of $18.20 billion, the company is well-positioned to handle a large influx of new government projects. AECOM's recently reported fiscal second-quarter 2018 results rose from a year earlier and surpassed estimates.

AECOM recently merged with rival URS to enhance its focus on the energy and transportation sectors, areas that are sure to see new spending under the Trump administration if the president can pass his infrastructure package. The stock has seesawed since Trump's victory, currently standing at $32.66 per share as of September 30, 2018, versus $27.61 on Nov. 8, 2016. The stock has a median 12-month price target of $38.63, suggesting a nearly 14% upside potential, but it earns a consensus rating short of "buy."

2. Vulcan Materials Company (VMC)

"Construction aggregate" sounds like a boring business, but these little bits of gravel, limestone and sand form the foundation of all sorts of building projects, including high-priority infrastructure items such as bridges, roads and industrial plants. Vulcan is the largest producer of these construction products, and it estimates that 75% of the population growth in the coming years will take place in states currently served by the company.

Vulcan has a market cap of $14.708 billion and generated annual revenue of $3.89 billion in 2017, which equates to a revenue growth rate of nearly 8% year over year. As of September 30, 2018, the stock was trading at $111.20, a year-to-date decline of over 12%. On July 31, the company reported lower-than-expected second-quarter earnings on higher-than-expected revenue; nonetheless, both earnings and revenue rose from the previous year, due to increased public construction activity. The company's forecast was for continued growth over the next 12 months. Analysts are expecting continued growth as well, despite the sliding stock price: the median 12-month price target is $139.75 (implying over 24% upside potential), and the consensus rating is "buy." (For related insight, read more about Vulcan Materials' inorganic growth.)

3. Martin Marietta Materials, Inc. (MLM)

Martin Marietta is a leading manufacturer of cement, asphalt and other building materials, with a strong presence in states such as North Carolina, Texas, Colorado and Georgia, that have booming construction economies. The company recently kicked off a massive project in Texas known as the Medina Rock & Rail, strengthening its foothold in the lucrative Texas market, and 2017 proved to be a record year in terms of the company's revenue, which hit nearly $3.97 billion; 2018 has been strong so far as well.

Martin Marietta is based in Raleigh, North Carolina, and it has a market cap of $11.465 billion. The company reported higher-than-expected second-quarter earnings and revenues, which grew over the previous year due to increased pricing and shipments. The stock currently trades at $181.95, down 20% year-to-date. The average 12-month price target of $249.42 suggests upside potential of 37%, and the consensus rating is "buy." (For more, read about Martin Marietta's bluegrass and aggregates businesses.)

4. Quanta Services, Inc. (PWR)

Quanta is a specialty contractor providing infrastructure services primarily to the oil and gas and electric power industries, all of which stand to benefit under Trump's infrastructure spending plans. Quanta is also a prime provider of oil and gas pipelines and renewable energy infrastructure, including onshore and inland hydroelectric projects.

Quanta has a market cap of $4.983 billion and posted annual revenue of $9.466 in 2017, up from $7.65 billion in 2016. As of September 30, the stock was trading at $33.38, down 12% year-to-date. Its 12-month price target of $46.50 leaves room for over 35% growth. The consensus rating of analysts is a "buy," and though the company saw a 1Q18 earnings miss, its revenues rose year-over-year that quarter.)

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