Week 5 - Models in Transp & Basic transp concepts and measures Flashcards
What are models in transp?
- models of physical transp system prominent part of transp planning tool kit
What are the types of models in TP?
i. diagrams
ii. physical models
iii. virtual models
iv. data models
v. GIS models
vi. framework models
Explain diagram models in TP
- 2D, elevations, cross sections
- considers flows of traffic (modes; #bike and auto lanes; auto, pedestrian, bike counts; directions of travel)
Explain mathematical models in TP
- express relationships mathematically
- general function relationship
- x explanatory, y is response
- ex. forecasting, regression
Explain network models in TP
- foundations of transp modelling and analysis are interconnected networks
- multiple ways to travel b/w nodes (redundancy)
- nodes and links
What are nodes and links?
Nodes: network locations, like airports, parking lots, intersections
Links: enable connections b/w nodes, like highways, airways, railways, etc.
- key consideration for both links and nodes is capacity
What are basic transp measures?
- capacity (C)
- throughput (Q)
- demand (D)
- density
- speed (V)
- design speed
- reliability
- safety
- environmental performance
- sustainability performance
Explain measure of capacity
Max number of units processed in unit of time
- ex. people/hr, vehicles/day, etc.)
Explain measure of throughput
Actual flow/volume of units in given time
- relates to productivity of infrastructure
- ex. people/hr, tonnes/hr, etc.
Explain measure of demand
Number of units desired per unit of time
- reflects collective decisions (actual and latent)
- latent demand (desire we’re currently unable to satisfy)
- typically, Q =< C
- if D > C, then Q = C
- if D < C, then Q = D
- expressed per time unit U (utilization)
What is utilization?
the percentage of capacity that is represented by throughput
- often depends on perspective (Ex. busy side of highway has better utilization than empty in service provider’s perspective, but opposite from user perspective)
- therefore, usually provide 85th percentile b/c good tradeoff b/w average and max
What is an alternative measure of utilization? (equation)
Volume capacity ratio (V/C)= (Q(people/hr))/(C(people/hr))
- critical factor in any capacity analysis is proportion of facility’s capacity being utilized by current or projected traffic
1. below capacity (v/c less than 0.75)
2. approaching capacity (v/c ratio 0.75-1.0)
3. exceeding capacity (v/c ratio greater than 1.0)
What is the key consideration in planning for networks?
- usage level the network is designed for (avg, max, 85th)
- balancing costs with levels of service
Explain measure of density
Amount of activity per unit area
Density = activity or items/area
Explain measure of speed
aka velocity
V = distance/time
- looking at range of speeds important, not just determining average speed
- max depends on physical and legal limitations