Note: The world has many words for its fascination with motorways, freeways, interstates, highways, expressways, autobahns, carriageways, autoroutes, and more. These are all assumed the same in this post.
Which is better: a street with no sidewalks, or a street with a sidewalk on every-other block? Or, is having a sidewalk every-other block no better than having none at all?
Let's talk grade separation.
One of the reasons that freeways are fast, efficient, and have such a high vehicle flow rate is because of their complete lack of interruption. They are free ways; they operate freely without other modes of transportation or crossings. In short, they have controlled access which also bans non vehicular modes of travel from entering, except at designated locations.
 |
| Overpasses are another prime example of keeping traffic separated |
We're all told when we are young to look left-right-left (or right-left-right, for left-side driving countries) before we cross the street. What is it that makes this necessary? The short answer: inefficiency. If you eliminate the road or provide a bypass/overpass/underpass, there is no need to stop, wait, and look both ways before crossing the street.
Transit, meanwhile, requires the same level of separation for similar efficiency to be achieved as freeways. Traffic can flow freely without interruption (aside from congestion due to insufficient capacity) whether you are on road or rail, provided it is dedicated its own right of way.
This holds true regardless of where you are or how you are getting about.
 |
| This train sitting in traffic in Portland, OR is no better off than a bus or car sitting in traffic |
 |
| Buses and trains in Portland, OR sharing space with cars, bikes, trucks, and pedestrians; this greatly slows the system down and makes it far less reliable |
Often times, this is not a black of white situation. While transit sharing space with automobiles is one of the worst examples, it can be significantly more cost effective to operate trains over their own space on the surface. Usually, the most important thing that can be done is simply removing cars. However, one of the reasons metros (subways) are so effective is because they operate without interference. When
passenger barriers are in place, as discussed in a previous article, transit doesn't even need to be concerned about pedestrian interference on platforms. This can be done with all modes of transit, even buses.
 |
| Casablanca, Morocco operates trams on surface streets which do not allow automobiles - only pedestrians |
Grade separation is important for many reasons, including, first and foremost, reliability. It keeps transportation on-time and on-budget. I use the general term "transportation" here because this applies to freight and passenger vehicular traffic, too!
 |
| What if the Sydney Harbour Bridge had a lift for ships to pass under? What a mess that would be! |
 |
| Any time you are suck waiting for a train at a grade crossing, think inefficiency |
The primary cause for grade crossings, whether you be at a red light, on a train, or floating on a river is one of cost. It is very expensive to untangle modes of transportation. Airplanes tend to be the mode of transportation entirely grade separated, except for a few places (Gibraltar's airport, for example, mixes airplanes and cars).
In general, the more dedicated right of way you have, the faster, safer, and more efficiently you will get to where you are going.
 |
| High speed trains, like Russia's Sapsan, operate just as a car on a motorway would - fast and reliably |
