Understanding patient flow in hospitals

10 Understanding patient flow in hospitals
Understanding how flow affects
performance
Everybody who travels experiences, and is part of, flow on every journey they make.
But heavy traffic, with managed motorways in place to maximise the number of
vehicles that can use a stretch of road, are commonplace. This is a long way from the
design vision of open stretches of motorway and free-flowing traffic.
We might imagine that the vision of an uncrowded road where travel at full speed was
possible was initially the case. But, over time, people adapted to the opportunity the
new roads offered by using them more. More people go to work that way, new factories
and warehouses are built close to junctions and, as a result, traffic increases.
With the limiting factor of the size of the road itself, this eventually leads to reduced
speeds. The way in which this happens rests on the concepts of complex adaptive
systems and the theory of constraints – which are vital for understanding flow.
We now introduce a simple model to illustrate this theory.
A simple guide to congestion
On a free-flowing motorway, typically the right-hand lane will be occupied by cars
travelling faster than the left and middle lanes. To travel safely at 70 mph, a car needs
about 100 metres of road. At slower speeds, less space is needed.
The maximum capacity of a kilometre of road, assuming safe distances are maintained,
is around 43 cars. The fastest cars will cover a kilometre in 32 seconds.
If more cars join the road, traffic will have to slow down to maintain a safe distance. In
the managed motorway example, this allows 51 cars (20% more) to fit in a kilometre
of road. The average journey time increases by only 4 seconds but total time for all
road users jumps by 32% (28 to 37 minutes) (Transportation Research Board, 2008).
Once more traffic joins the road, speeds must reduce further. At 30 mph, all of the
lanes travel at the same speed, and three times as many vehicles can fit in a kilometre
of road (see Table 1 for a breakdown of this). However, the average time taken to travel
1 km has almost doubled and the total time for all users has increased almost six times.
The fact that more congestion in a confined space means less speed, and less flow once
a certain level of occupancy is reached, is well established in modelling of road traffic.
Heavy congestion cannot be solved by asking people to drive faster. In fact, speed can
only be increased by reducing congestion first.