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
Explain measure of design speed
Design physical system to encourage travellers to travel at design speed (road geometry)
- wide curves/lanes for higher speed and tighter for lower
Explain measure of reliability
Commonly used in reference to the level of consistency in transp service for a mode, trip, route, or corridor for period of time (making trip on time, every time)
- people remember when travel times take longer due to unexpected delays
- just in time (JIT) production depends on reliable delivery times (scheduled economy)
What factors influence reliability?
- incidents: collisions, break downs, disruption of normal traffic flow
- work zones: construction/maintenance
- weather: full range of events - low visibility from fog, rain, snow, sun in eyes
- fluctuations in demand: day-to-day variations caused by changes in activity levels (ex. weekends)
- special events: cause dramatically different travel patterns/volumes in vicinity
- traffic control devices: poorly timed signals or periodic signal events (ex. railroad crossing)
- inadequate base capacity: normally congested roads more susceptible to effects of other factors
What are some approaches to measuring reliability?
- probability that actual minus expectation is less than a certain threshold over certain percentage of time (threshold depends on trip purpose and how willing you are to accept being late)
- buffer time index: represents extra time that travellers must add to their avg travel time to ensure on-time arrival
Explain measure of safety
Measure frequency of accidents per person and the distance they travel
- depends on amt of driving/travelling you do to establish risk exposure
(# injuries OR # property damage OR # fatalities)/(person/km OR vehicle/km)
Explain measure of environmental performance
Typically impact per unit of transp activity.
- ex. emissions of GHGs
- measure impacts based on transp output
- carbon footprint calculator
- most common effort is to reduce GHGs
Explain measure of sustainability performance
3 pillars: social, economic, enviro
- driven by society b/c demands tied to liveability and health
What are enviro principles under the sustainability performance for TP?
- renewable, inexhaustible, recyclable resources should be favoured
- transp with lower energy intensity should be favoured
- emissions from vehicles that compromise air quality should be reduced
- protect ecosystem integrity
What are social principles under the sustainability performance for TP?
- human health/safety promoted
- everyone should have affordable and equitable access to jobs, schools, services, etc.
- transp systems should be convenient, comfortable, sensitive to disabilities, etc. (generally people-friendly)
What are economic principles under the sustainability performance for TP?
- transp decisions should be compatible with society’s ability to pay for full costs (externalities)
- supportive of vibrant economy
Why are principles for selecting performance measures important?
- connect to importance of monitoring and eval
- regardless of transp system area planning and operations, performance measures and indicators matter
- key role in decision making, understanding and communicating with stakeholders
What are principles for selecting performance measures?
- comprehensive: reflect various economic, social, and enviro impacts, and various transp activities
- data quality: data collection practices should reflect high standards to ensure accuracy
- comparable: data collection should be standardized so results are suitable for comparison
- easy to understand: indicators must be useful to decision makers and understandable by public
- accessible/transparent: indicators and analysis details should be available for all stakeholders
- cost effective: cost effective to collect
- net effects: indicators should differentiate b/w net impacts and shifts of impacts to diff locations/times
