The Mudcat Café TM
Thread #165036 Message #3956309
Posted By: DMcG
12-Oct-18 - 12:46 PM
Thread Name: BS: 'Not so Smart' Motorways.
Subject: RE: BS: 'Not so Smart' Motorways.
Thanks for that, Mr Red. Traffic management is one of the vast array of subjects I know nothing about, and of course you are right that one definition of the capacity of a motorway is when every square metre of the tarmac is covered by a car. Clearly you can't get any more cars on there, so you could call that the capacity. But that does seem to omit any idea that the people in the cars want to get somewhere, and if everyone was stopped the capacity would be the same as the less likely position where everyone is moving at the same non-zero speed.
Here is one paper that uses a different definition, which I have copied below.
1.1.2 Capacity The definition of capacity varies depending on the context applied. Two distinctly different definitions are attributed to the terms “flow rate” and “maximum number of vehicles”. The Highway Capacity Manual (2000) defines capacity as the following:
“The maximum sustainable flow rate at which vehicles or persons reasonably can be expected to traverse a point or uniform segment of a lane or roadway during a specified time period under given roadway, geometric, traffic, environmental, and control conditions; usually expressed as vehicles per hour, passenger cars per hour, or persons per hour.” (Transport Research Board, 2000, Chapter 5)
This definition presents an idea of throughput while accounting for the various factors of friction that inhibit vehicles. By comparison, the literature in the VISSIM manual (PTV, 2007) refers to flow rates as a “saturation flow”. The term of saturation flow in the Highway Capacity Manual specifically refers to signalised intersections and available green time (Transport Research Board, 2000, Chapter 5). Throughout this paper, the term saturation flow as identified in the VISSIM manual is applied as the maximum traffic flow volumes that can be achieved over an hour long interval. Capacity will be referred to as the maximum achievable throughput given the constraints that limit the traffic flow, such as weaving, grade, traffic composition and geometric bottlenecks. The variation between the two terms is that saturation flow refers to potential hourly flow volumes if the inhibiting factors were not present (under ideal conditions). The other definition reflects an achievable throughput that is subject to the constraints in the network. The capacity of a section of freeway could therefore be significantly less than the saturation flow.
Hardly enthralling reading, but in short the definition they use is about flow, which seems more reasonable to me.
Using the idea of flow, it seems to me that the objective of the smart motorway is to increase the flow, but that it does so in a way that greatly increases the risk of turbulent flow - caused by obstacles andlane-changing amongst other things. THey have tried to increase the total number of vehicles on the motorway at the cost of increasing this risk.
Conversely, you can increase flow by eliminating the sources of turbulence, which are the kind of things Steve.
I am grateful to the OP and various commentators that have persuaded me to read up a little on this subject.