Road networks in cities have become works of art with their intertwining nature, brutalist concrete architecture and the shimmering red lights from increasing traffic congestion.

Attitudes have also evolved as traffic becomes a normal part of daily life, for instance, sociable Sao Paulo drivers are known to interact with each other, in some cases flirting for a date.

However, despite this bright side of traffic, congestion is also a drag on economic growth, for instance, a World Bank study found traffic in Cairo costs the economy about four percent of its GDP.

The intensity of traffic congestion varies around the world according geography, population density, lack of proper urban planning and level of car sales.

GPS system TomTom uses its live database to create an index on traffic congestion across cities worldwide; at the time of writing, Mexico City ranks the worst with the average journey time taking 66 percent longer than it should, followed by Bangkok and Jakarta with additional journey time-spend of 61 percent and 58 percent, respectively.

Such cities, in 2016, have some of the highest population density rates at over 2,600/km² and 5,500/km² for the metropolitan areas of Mexico City and Bangkok respectively, and over 15,500/km² for Jakarta city; while car sales in these cities has been supported by improved median household income levels rising at an average rate of over five percent annually over the 15 years.

Interestingly, Dhaka city in Bangladesh, with an outlier population density at over 43,350/km² in 2016, is already widely notable for its horrendous traffic, but the situation will only get worse as income levels improve and government strategy plans continue to be delayed.

What can be done?

The solution revolves around supply and demand: either increase road capacity or reduce the number of drivers — even better, do both; but as evidence shows, it is not that simple in reality.

In London, the introduction of the congestion charging zone in 2003 resulted in a decline in traffic, however, in recent years we have seen the number of private hire drivers (e.g. Uber drivers) rise dramatically (reaching over 100,000 drivers in 2016, from 59,000 in 2010).

Demand cannot be easily stopped; car sales keep increasing as median household income rises, especially in emerging cities across India, China, Indonesia, Sri Lanka and Mexico.

Elon Musk, founder and CEO of SpaceX and Tesla, recently vented on twitter about his daily commute through LA.

The solution he explains is to go underground.

Just like SpaceX, he plans to meet demand by creating supply of cost-effective equipment (specifically tunnel boring machines, or TBMs) with his new start-up company (called The Boring Company).

The cost of building tunnels is impressive.

GlobalData compiled a small sample of road tunnel projects and their costs in cities across the world and found on average it costs over $800m per mile to complete.

The sample contains tunnel lengths ranging from 0.5 to 13 miles, with budgets ranging from $4m (for underpasses) to over $14bn for the famously expensive and continuously flawed Central Artery/Tunnel Project, nicknamed Big Dig, in Boston.

So the cost of building tunnels varies greatly according to geography, length, equipment requirement and specifications, nonetheless investor risk does have some cushion as a significant portion of the cost in most cases is covered by governments through loans or subsidies.

It seems that for many major cities in developed countries such as London, Singapore, Tokyo, Sydney, Stockholm and Istanbul, the best way to go is down.

The strategy is to build more tunnels to upgrade and improve rail, road and utilities rather than deal with the risks of re-structuring existing surface networks. For now, tunnelling seems the most realistic solution to solve traffic congestion in built-up areas over flying cars or self-driving vehicles.

The benefits are also very appealing: reduce carbon emissions, increase public space, reduce light and noise pollution and, as conspiracy theorists might add, provide safety from World War 3.