Energy from cigarette butts, a retractable pitch & multiple arenas - the NFL stadium innovations Man Utd could imitate for revamped Old Trafford

The Allegiant Stadium is the NFL's newest arena and became the new home of the Las Vegas Raiders in 2020 after the franchise moved from Oakland to Sin City. The stadium cost an estimated $1.9 billion (£1.5bn) to build, and its construction took just under three years.

Its most notable feature is a retractable pitch, allowing the two teams who call it home, the Raiders and UNLV Rebels, to play on different surfaces. There is a tray underneath the stadium, weighing 9,500 tonnes, which is called into action when the surface needs to be switched.

The tray is powered using 72 electric motors, moving one pitch outside the stadium and the other into it. While the astroturf surface - favoured by the UNLV rebels - is being used, the grass field can receive natural sunlight and water.

The Tottenham Hotspur Stadium also has a retractable pitch so it can host NFL matches as well as Spurs games. United could use a retractable pitch if they ever wish to rival Tottenham in hosting NFL games or hold other sporting events there.

Old Trafford already hosts the Super League Grand Final each year, and in the past the match has left the turf in a rotten state. Having a retractable pitch could prevent that from happening in the future.

Yes, really. This is another feature of the Allegiant Stadium. Since the arena opened, more than 60,000 watts of energy have been created thanks to its cigarette waste-to-energy collection programme. The discarded cigarette butts are put through a combustion chamber to collect the ashes, which in turn are used to create power through a generator.

As well as creating energy, the programme ensures the cigarettes do not get into the waterway and it reduces litter in the local area. And it has helped the stadium achieve top-tier sustainability status.

Old Trafford is a smoke-free stadium, although fans still smoke outside it. However, with many supporters' smoking habits switching from cigarettes to vaping lately, this programme might struggle to get off the ground in Manchester.

When United arrive at the NRG Stadium in Houston to play Real Madrid, they will be flanked by sports arenas from all angles. The Astrodome, home to baseball World Series champions Houston Astros, is adjacent to the stadium, and just around the corner is the NRG Arena, an indoor venue which hosts livestock shows and rodeos.

In the US it is quite common to have different sporting stadiums in the same vicinity, and the sight is also familiar in Manchester after United's rivals City completed work on their new training ground in 2014, moving from Carrington to just a stone's throw away from the Etihad Stadium.

As well as being the place where Pep Guardiola puts his side through their paces each day, the City Football Academy houses the women's and youth teams' stadium and training pitches, giving the club all sorts of logistical advantages.

United have floated the idea of constructing a smaller stadium next to the revamped Old Trafford which the women's team and academy sides could call home. That would allow them to move from their current site at Leigh Sports Village, which is a 40-minute drive from Old Trafford and very difficult to access via public transport.

The NRG Stadium and the Allegiant Stadium both benefit from air conditioning, although the latter's system is more modern than the former, which was built in 2002. The Las Vegas stadium is a fully enclosed, climate-controlled dome, and as well as air conditioning it provides heating, air purification and ventilation.

The air conditioning will come in handy when United play Madrid in Houston, with temperatures set to reach 34 degrees Celsius, and especially when they face Dortmund in Las Vegas, where it could be as hot as 44 degrees Celsius.

With Manchester experiencing one of the wettest and coldest Julys ever this year, air conditioning is unlikely to be a priority at a modernised Old Trafford. But then again, in this age of climate change and ever rising temperatures, you can never be too sure.

Stadiums in the U.S. are characterised by having large and comfortable seats, especially when compared to old English grounds such as Old Trafford. While the Emirates Stadium and the Tottenham Hotspur Stadium have got with the times, seats at United's ground remain terribly tight and it is very difficult to avoid making contact with the person sitting next to you.

Fans also often complain of long queues for refreshments and tight space in the gangways and corridors, which is not something U.S. sports fans often have to contend with as the customer's comfort is prioritised.

Another problem Old Trafford has is the quality of views. Though the pitch is visible from every seat, in many sections you cannot see the rest of the stadium due to its cantilever roof, a legacy of expanding the Sir Alex Ferguson stand first in 1995 and then the Stretford End and East Stands in 1999. In the Met Life, Allegiant and NRG Stadiums, all corners of the ground can be seen from every seat.

While United fans might wish for the club to take inspiration from some aspects of the U.S. stadia Ten Hag's side are playing in on the tour, there is one area they hope officials will not be paying attention to: ticket prices.

For the friendly against Arsenal, the cheapest tickets on Ticketmaster were priced at $171 (£132), while seats behind the goals cost around $300 (£236). The situation is slightly better for the game against Real Madrid, with the lowest-priced ticket being $99 (£76) and the best non-executive seat in the house costing $365 (£282).

United fans are disgruntled about season ticket prices rising next season, but it is still far more affordable to follow the team in England than in the U.S. Tickets for the friendly against Lens at Old Trafford in August are only £20 ($25), while fans can attend Premier League matches for as little as £36 (£46).