If you have a city that struggle to grow past a certain population cap, the fundamental cause is most likely traffic. Without a smooth traffic your city cannot provide rubbish service, raw material cannot get to your factories and factories cannot get their products to your commercial zones.

Then next thing you will have business going bankrupt, people have no work and they decide to move out. Then your citizens start dropping dead and the hearse cannot get to them and the entire city is littered with trash and dead bodies.

Entire city shuts down, everything goes bonkers, trucks can't ship fuel to your oil power plant, and those wind farms just couldn't cut it, sewerage has no power to pump poo out and you close the game.

The common misconception is to point it entirely at bad game mechanics, but let's face it - it's your terrible traffic management that that was left unchecked, and you are just looking at the cumulation of your mismanagement.

If everyone has up to the minute traffic data and computational precision to calculation whether it takes less time to just get in the traffic and wait it out or cut through the back streets like Hollywood movies, your local politician would probably have one less topic to bicker about at election time.

Mitigation
tl;dr This is actually the result of someone doing every bit of traffic management wrong. Follow the guide and implement solutions, you have too many traffic, simple problem, it's not "game design" - it's YOU.

Vehicles in CS heads to a certain destination for a reason, if you have 100 cars stuck on the highway exit it means that exit is closest to their destination, maybe your residential zone sits between your industrial zone and the city exit; or one exit is simply not enough. Wishing the vehicles travelled on different lanes doesn't fix the issue, if they actually did you will just have a bigger jam at the exit.

By default most intersections uses traffic lights, however the cims are perfect drivers and do not crash, turning off traffic lights alone will do wonders for your traffic. See screenshot below on how to see them, simply click on the intersections' traffic light icons to turn them off.

Only instance where you should use traffic light are intersections with heavy traffic on all sides, as traffic lights give the heavy traffic from all sides to turn unobstructed.
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Author's note: Road hierarchy is a method of planning you implement via design not some function intergrated into the game.
New players have the impression that simply having bigger roads should solve all traffic problems, the reverse is true. Having bigger roads does not change the destination of your traffic, and that four lane road nobody drives on is not going to solve your traffic problem when there's just one bridge linking your 50,000 residential zone to the industrial zone.

In actual city planning, roads have different hierarchy assigned to them to dictate what development is permissible in the area. In Cities Skyline it's enough to separate them into just three:

• Local Roads - Streets, Cul de sacs (aka court/dead end), that local business and residents access, provides access to properties.
  
• Collector Roads - Collects and distributes traffic between your arterial roads and local roads through traffic management
  
• Arterial Roads - Your main roads where intra-city traffic takes place with uninterrupted traffic flow and minimal to no direct property access

Building roads with no consideration of road hierarchy, i.e. linking suburban roads onto expressway and let one car slow down 20 cars when it's turning in = bad road design. It's important to identify your road hierarchy and not mix them up, and if you have to, assess the potential impacts of breaking that hierarchy.

Mitigation
Establish road hierarchy early, and avoid using biggest roads as your local roads, not only this increase your outgoings, you will also lose considerable building space by placing roads bigger than the local traffic requirement.

Using appropriate roads will also save you money (if you are not using unlimited money option) as the cost between a boulevard and a two land suburban street is very different.

Waste management facilities, airports, cargo airport/station/port produces a lot of traffic, and it's important that you provide those building with dedicated roads and not overload the area with residential/commercial/industrial (RCI) buildings.

Mitigation
Avoid placing traffic heavy facilities on arterial roads and collector roads.

The cargo port above uses a one way road to lead the traffic from industrial zone in and sends them back out the other, if the one way roads are switched, the vehicles coming in would be butting heads with vehicle exiting the port. It's important to make sure your road design is taking consideration of left hand/right hand roads and which side is experiencing heavier traffic and design exits and entrance accordingly.

Mitigation
Use the traffic tool to find out where your vehicle traffic is heading, it they are crossing path on a highway you will need to redesign the exits.

Pretty self explanatory. A bus in Cities Skyline (CS) carries 30 passengers (pax) on standard budget. The bus itself takes the space of a truck on the road, if all of your 30 pax have no alternative but to drive to work, that's a whooping 30 cars on the road versus just one bus you have to deal with.

Mitigation
Provide public transport suitable for your city - if your city is very lightly zoned there's no need to build subway as the first type of public transport.

Having a bad public transport can do more harm than good to your traffic. Buses, despite having a capacity of 30 pax, if the line is overloaded with buses that are empty or have very little passengers, they will only manage to take up spaces.

Another mistake is not interlinking your public transport network, or have just one mode of transport and wonder why it's not helping. The citizens in CS will happily take a bus, transfer to a tram into your commercial district and get on another bus there to go to work.

Mitigation
Have transit lines that runs in local area and interlocking them with other transit lines/transportation instead of having just buses running from north side of the city to the south. In the screenshot below, locals are collected by trams and each tram line has one or two shared stops with adjacent lines of another circuit as well as inter-city subway and monorail lines.

Most of the times, having interlocking transit lines would suffice, inter-city transit system can come later, most people do it the other way around, they provide inter-city transit but the local population has no access to public transport, so they end up driving anyway.

If you are having issues with too many public transit vehicles, open up the traffic panel (see image) and inspect all of your transit lines, consider reducing the vehicle count modifier that line which will in turn reduce the number of vehicles on that line. Default vehicle count is based on distance, and may not reflect the actual needs of the area it travels.

Industrial zones will need to import from outside of the city, then the product is either shipped to your local business or out of the city. Most of the times, they go out of the city, hence having industrial zones far away from inter-city highways, forcing them to drive through residential zones will always guarantee your industrial zones falling apart.

Mitigation
Industries can function with one of the following to import and export

• Highway going outside of the city map
  
• Cargo train stations
  
• Cargo seaport (doubles as cargo train station)
  
• Cargo airport

Building one of the above near industrial zones that's too far can solve your truck problem, the factory trucks will ship their products to one of those exit points instead of driving all the way to to the inter-city highway, and is a more cost effective alternative than building a new intercity highway.

Having an arterial road that goes directly from your industrial zone to the highway however, is always better than letting them drive through the residential zones.

Suggested by Sunwave
Cims like to take the shortest route to get business done. So if you build a intra-city highway while the nearest road to work is through that two land road, the cims will take that route and cause traffic jams.

Mitigation
Chance of the entire main roads being thrown away in favor of the small road is small when you utilise road hierarchy and avoid creating small shortcuts. If you decide to create one for aesthetic reasons, consider limit the traffic to local only or ban trucks.

The extra dollar provided by having a toll booth may outweight the overall economic benefits of having a smooth traffic. If you have a toll booth which is causing traffic jam, consider removing it, the congestion may not worth the $250 it's pulling in.

This actually decreases your traffic speed and will also contribute to congestion.

While the solution may appear self explanatory, it's often ignored when it comes to highway intersections, if you have a three lane, one way highway leading into a single lane highway exit, it's going to jam because your traffic is funnelled from a three lane road into a one lane road at super high speed, while having a three lane may not be possible through upgrade, consider upgrading the one lane exits on your highway to a two lane, one way highway instead.

CS traffic can be easily managed through appropriate city designs, 80%-85% traffic is pretty easy to accomplish, not to mention the extra tolls and public transport ticket fees you can collect.

Always look at the solutions, anyone can look at problems and scream at it - and the problems will remain.