In SimCity, there are two different types of road. These are streets and avenues.
Streets are one square wide and avenues are two squares wide. All streets are the same width, so a dirt road can be upgraded to a high density street using the upgrade road tool.
In order to upgrade a street to an avenue, however, you need to destroy the old street and replace it with a larger avenue, which will remove all infrastructure that exists along the old street.
If you're in a situation where you already have the maximum capacity version of a particular style of road but don't have the space to replace it with an avenue, there are other options available. It is possible to build additional roads within the vicinity to alleviate the amount of traffic travelling down your currently over-crowded road. To do this, bring up the traffic data layer to see where all of the traffic is coming from, and build new roads that intersect the busiest streets to provide additional routes for your sims to travel down. The traffic data layer can be accessed using the 'All Data Maps' button in the bottom right of the interface, or by toggling the 'upgrade road' option.
In addition, there is the possibility of setting up mass transit. Mass transit works by building a depot (and adding additional trucks as required to increase the number of sims that can travel per day and reduce the wait time for busses) and placing stops. The region that a stop will cover is highlighted along the roads as you place the stops. For the maximum effect you will want to place stops along your busiest streets (see your data layer for traffic again) and ensure that the wait time for passengers is managed to ensure it doesn't get too high.
You can monitor wait time by bringing up the information screen for your mass transit depot, and you can monitor the amount of traffic a particular stop is attracting by bringing up the information screen for each individual stop.
I do not yet know whether the forest is anything but cosmetic, but from the land value map it looks like the trees do not do much of anything, although in SimCity 4 wild vegetation was an important tool for increasing desirability.
To plant or cut-down the forest one needs to select Parks > Nature (§§) then scroll to the right where they would find the tools for the job, as seen on the screenshot below.
There is no immediate reaction to the use of the tool (except for the money leaving one’s account in an instant, much like EA pre-order), but I suppose it grows over time (much like Origin’s delivery of pre-ordered titles on release day). That said, try as I may, I had not had any tree show up anywhere I had planted, so this tool still needs figuring out.
The above quote is wrong, trees do show up immediately, but not always. Still do not know if it is a feature or a bug.
Apologies for strange and foreign tongue as per interface. I am yet to figure out how to change it for demonstration purposes.
Best Answer
Parks do not increase the happiness of industrial buildings.
Parks increase land value. Land value causes residential and commercial buildings to convert into wealthier versions, but only happiness will cause density upgrades. You can check happiness in the happiness data map.
To upgrade the density of the industrial building, you need to improve its happiness (aka profit) through the freight system:
First, freight must be produced. This happens when the building's needs are met and it is open. The number of workers doesn't matter as long as the building is open.
Second, freight must be delivered. Each hour, a freight truck is spawned from each industrial building (unless one is already out). The freight truck will carry freight to a building with available freight orders. The truck will exchange the freight for profit (1 for 1) and reduce the available orders in that location. The truck will then return with the profit and drop it in the building.
Buildings with freight orders include: freight warehouses, commercial buildings, many specialization buildings (such as the mine). If freight is delivered to a commercial building, it will also give a bonus to that commercial building's happiness.
Once delivered, freight decays over time and orders replenish over time.
Third: once happiness reaches the threshold, you also need sufficient room and road type to allow the density increase. Low -> Medium requires space behind the building - the building is twice as long. Medium -> High requires space (or other industrial) along the road - the building is twice as wide.
It may also be helpful to know that High density Industrial buildings will burn down without the hazmat firefighting addon.
If you're still confused, I've talked about freight and goods as related to happiness and density before.