the need to get from A to B is something
we're all familiar with in the u.s. most
people spend about an hour traveling to
and from work not to mention all the
other sorts of traveling we do but
despite all the work engineer's put into
making those journeys go smoothly it
seems like things go wrong more often
than they should maybe you've been
delayed at the air for encountered
signalling issues on the subway or got
caught in a traffic jam while the
weather often plays a role one of the
biggest reasons that transportation
systems run into issues is that they
have to deal with another persistent
difficult unavoidable element us if you
want to design a good transportation
system you have to understand the people
who are going to use it
transportation engineering is one of the
largest disciplines of civil engineering
every Highway railroad shipping lane an
airline route relies on transportation
engineers to guide the way suppose you
were tarik setting up a subway system
for a city you'll need to plan design
and construct every last element before
it can function what's more
transportation systems are almost by
definition heavily connected every piece
needs to support all the others if no
use having state-of-the-art trains on
brand-new tracks if the stations aren't
built where people need them and once
the system is up and running you'll need
to constantly monitor how it copes with
demand that will help you make decisions
like how to improve access to stations
and how often to run trains the same
kind of thinking applies to plane race
and shipping systems although roads and
highways have some extra considerations
the physical aspects of engineering
we've learned about so far play a big
part of the design work you need to
build components out of the right
materials and put in the right safety
features for example you'll want to
build a Subway's tracks with enough
mechanical strength to avoid buckling
because of an overloaded train and the
key element in transportation
engineering is people we're a core part
of the system unlike other disciplines
we've looked at this kind of engineering
isn't about processing in animal
materials into something that we use or
consume it's about moving people in a
way that's safe convenient and
comfortable that means considering
social requirements in addition to
physical ones for instance how do you
decide where to build tracks and which
locations they should connect ideally
you'd build enough infrastructures to
support every journey that anyone would
want to take but like everything else in
engineering cost is one of the biggest
constraints you have to work with since
you probably have a fixed budget for
setting up the system you need to
prioritize what gets built and where and
to do that you have to understand what
kinds of journeys are going to be made
so with a subway some people will want
to visit their friends or go shopping
while others are just trying to get to
work the layout needs to support those
different needs while making enough and
passenger fares to keep the system up
and running so the social and economic
goals of your city have a big role in
the design of the network in general
transportation engineering is often
about taking a bunch of different
priorities and combining them in the
best but rarely perfect way and that
applies on every scale your new subway
system for exam
might have a point where two lines merge
into one taking passengers from
different starting locations to the same
stop but if two trains approach at the
same time how do they both pass through
obviously no one wants a collision so
your job as an engineer is to design a
system where the odds of that happening
are as small as possible since we're
dealing with heavy objects traveling at
high velocities that reason need to be
designed to allow changes to those
velocities while in advance while a car
traveling at highway speeds can safely
stop over a distance of 100 meters by
applying the brakes a passenger train
traveling at the same rate needs over a
kilometre before it comes to a halt
that's also why air traffic controllers
have to constantly monitor and
communicate with planes that are flying
at similar altitudes because the
passenger plane can't be maneuvered
quickly the flight paths need to be
designed to ensure that flights from
different locations overlap as little as
possible schedules need to keep them out
of each other's way while also avoiding
turbulent weather but to avoid
collisions that your train Junction
you'll need signalling to give each
train enough notice when it has to stop
you'll need regular signals their
intervals along the track those
intervals are also known as block
sections signals give useful information
about what's happening on the tracks
several sections ahead like whether
there's another train or if it's safe to
travel at full speed to allow enough
time for the train to respond and stop
block sections tend to be no shorter
than the necessary stopping distance all
the signalling systems rely on a set of
lights above or beside the tracks that a
driver can interpret and respond to for
your brand new subway you'd probably
build in wireless signalling
capabilities to relay information
straight to the train cabin to the
driver or an automated system if two
trains are approaching the same Junction
signaling would tell one of them to slow
down several blocks sections ahead while
allowing the other to pass through and
that choice can be made in a few
different ways if one train is slightly
closer to the junction it might make
sense to give it priority while the
other is told to wait so if the other
train was delayed earlier on its route
you might want the signals to give it
priority even if it's further away to
help it get back on schedule or if you
want to get as many people to their
destinations as quickly as possible you
might let the train likely is to be
carrying more passengers through first
the details will depend on the
particular zuv the routes and the needs
of your system railway signalling
involves managing a whole network modern
approaches
also sophisticated enough that safety
mechanisms like electronic sensors will
automatically stop a train if the
signals indicate danger ahead when
they're working the combination of
signals and tracks give trains a big
advantage they go where they're supposed
to when they're supposed to that many
people mainly get around with vehicles
like cars trucks and motorbikes in
Britain for example about two-thirds of
all journeys are taken in an automobile
with every driver free to move
independently roads lose much of the
predictability that railroads enjoy
potential collisions aren't confined to
known junctions they could happen every
time someone changes lanes and even if
accidents don't happen all that's
switching back and forth causes
something we're probably all familiar
with traffic jams to understand why it
actually helps to think back to fluid
dynamics in general a highway system is
well designed if as many cars as
possible are flowing through the system
as fast as safely possible so on a given
stretch of highway you can think of the
efficiency of the system as the flow
rate of cars the flow will be the
product of the density of cars passing
through and their average speed much
like the flow of water through a pipe
except the driving force here isn't
water pressure instead is every drivers
individual desire to get where they're
going if there's only a few vehicles on
the road is not a problem the cars don't
have to fight for lane space or slow
down because of other drivers everyone
cruises along of roughly the speed limit
that's what engineers call normal flow
in normal flow you can increase the
total flow rate just by adding
additional cars in other words more
people music the system and the highway
is doing its job add too many cars
though and the problems start in normal
flow the extra cars don't affect the
average speed of each vehicle but once
the density of cars reaches what's
called a critical point the lack of
space between cars starts causing
restrictions everyone drives them little
slower to accommodate the other cars
around them add even more cars when you
start getting stoppages when one car
slows down or stops the others behind it
how to slow down - and so on this brings
down the average speed of all the cars
on the road even though there are more
cars the flow rate actually decreases
past the critical point because their
average speed has dropped so low that's
what engineers call force flow
everyone's desire to get ahead actually
worse each car trying to desperately
squeeze through every opening makes it
harder for the jam to ease up and for
the cars to flow steadily again there
might be other routes drivers can use to
get to their location but in general
every driver wants to take the shortest
route which seems pretty sensible on the
face of it but when everyone uses the
same route forced flow occurs you might
think that building another roads or
adding more lanes would improve things
in practice though most people still
take the route they think will be the
fastest which only makes things worse
so highway engineers tackle problems
like this in sometimes unusual sounding
ways one solution comes from German
mathematician Dietrich braze instead of
adding more rows you can actually
improve traffic flow by taking them away
that might seem counterintuitive which
is why is called brazzers paradox after
all how can the flow be improved by
removing a highway raises paradox worse
because when the seemingly fastest route
is taken away drivers end up using
several alternative routes instead even
if there's alternate routes are longer
they only take a fraction of the cars
each that keeps speeds higher and the
flow rate closer to normal on average
people will even get to where they're
going faster on the longer route of
course that's just one solution that
only works for some highways in general
you need to accurately model the flow of
traffic and develop sophisticated ways
to keep it flowing many of the same
principles used for signaling on
railroads
carry over to programming traffic lights
and even the design of roads themselves
the way rows merging Connect has a big
impact on how drivers act and the
eventual flow patterns in the future if
fleets have driverless cars tapes of the
road it could require whole new
principles of highway design much like
automatic train networks did for
railways but even if humans won't be
behind the wheel transportation
engineers will still be there ensuring
that those journeys are designed to be
safe fast and convenient in this episode
we looked at transportation engineering
we saw how transport systems are
designed from the outset what decisions
need to be made and how to operate them
with the help of signaling we learned
how systems like highways are built
around the behaviors of their users are
considering traffic flow let us assess
our highways efficiency and ideas
transportation engineers
use to improve it next time we'll
explore how engineers create
communication systems like the one
you're using right now crash course
engineering is produced in association
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