Transportation Planning Flashcards
Transportation Planning
a detailed specialization within the planning discipline. Happens at multiple scales- national, state, regional, citywide, neighborhood and site level
Trip Generation
Deals with the number of trips that a particular site is likely to generate. Thus, it is a byproduct of land use and intensity of use, factors which “induce” people to travel. The propensity to make trips is also dependent on the characteristics of the journey, trip purpose, and socioeconomic characteristics of the person making the trip (income, age, auto ownership). Trip generation rates can be defined and determined in a number of ways.
Origin-Destination Survey
a special type of survey that is often used to determine trip generation. An origin-destination survey will set up roadblocks along major routes. the imaginary line that denotes the boundary of the study area is known as the cordon line. Motorists within the cordon area can then be sampled and asked questions on where they are coming from (address or point of origin) and where they are going (destination). A more detailed survey with questions on socioeconomic characteristics can also be given to those sampled. The questionnaire is generally mailed back by the respondent.
Cross Tabulation Models
Can also be used to estimate trip generation. They allow for estimates of trip generation rates based on land use type, purpose, or socioeconomic characteristics. Needless to say, trip generation estimates based on current data become less and less valid with age. When local surveys are unavailable due to time or monetary constraints, published rates are used to derive estimates. Trip generation models’, tables and surveys all have their own sources of error and should only be seen as estimates.
Some typical Trip Generation Rates:
10 daily trip ends for every 1,000 square feet of general office space
9 daily trip ends per single family residential dwelling
7 daily trip ends per apartment unit
38 daily trip ends per 1,000 square feet of shopping center space
5 daily trip ends per 1,000 square feet of light industrial development
Note that trip end simply refers to the origin or destination point of a journey
Trip Distribution
Examines where people are going. A region or area is often divided into traffic zones. Trip distribution information generally provides information on how many trips are made between each zone and every other zone. The trip distribution component of the planning process also provides information on trip distances, time and cost, the nature of the trip, socioeconomic characteristics, and the nature of the transportation system.
The Gravity Model
Can be used to provide trip estimates based on proportional attractiveness of the zone (the “gravitational pull”) and inversely proportional to the trip length.
Modal Split
Deals with how people get to where they want to go, and the form of transportation that they use. By having information on the number of people using cars, mass transit, bikes, or walking, planners are able to estimate how many vehicles need to get from one place to another.
AADT (Average Annual Daily Traffic)
The amount of traffic on a roadway in a 24 hour period, averaged over a year
Peak Hour Volume
equals the hourly traffic during the peak period
Seasonal Hour Volume
the peak hour volume during different seasons
Design Hour Volume (DHV)
The capacity of the roadway to handle traffic.
Traffic Assignment
also known as trip assignment, allows us to use network models to predict the distribution of traffic for each roadway (the routes that will be used), by the hour. Peak volumes can then be compared the DHV to see which, if any, roadways are going to experience traffic over their design capacity, ie.e., where the congestion is going to occurr
Vehicle Miles Traveled (VMT)
a measure of vehicular mobility obtained from travel inventories. VMT is a function of many factors, including topography, population density, travel distances between home and other daily destinations (such as work, shopping, and recreation), and the availability of mass transit. Communities vary in their mix and in the significance of these factors.
High VMT indicates that more vehicles are on the road to meet growing employment, errand and other travel demands. It can also mean that the trip origins and destinations are getting farther apart and travel times are becoming longer.
Road Design
Focuses on everything from the nature of street hierarchy to design guidelines for local streets. Transportation engineers use “functional classification” to group streets and highways into classes, or systems, according to their character of service: local or residential streets are designed to serve only local land uses, collectors funnel traffic from local streets to arterials, arterials are major through roads that carry a large traffic volume. Arterials are often divided into major and minor arterials and rural and urban arterials. The functional classification system has been criticized for failing to consider the context in road design (other than urban and rural).
The origin of most of our current roadway standards can be traced to Federal Highway Administration studies following World War II. Definitions of “good” standards were based on “new” subdivision designs. These standards are often too wide for most local streets snad are only advantageous if there is a lot fo traffic and no off-street parking.
Typical Local Street Standards
500-foot maximum tangents
Use of stop signs or speed bumps to reduce vehicle speed
150 feet between intersections
Clear sight distances of 75 feet
As street classifications change, so do the standards. For example, the tangent and curve radii are supposed to be higher on collector streets because clearer sight distance is needed at higher speeds.
Autonomous Vehicle-Related Improvements
Only 41 percent of US roads meet the requirements for a “good ride”, as scored according to the international roughness index. deteriorating roads aren’t only a nusiance for human drivers: they are major impediments for autonomous vehicles (AV’s). To promote higher levels of autonomy, mapping software must be highly accureate- for difficult intersections, down to the inch. Even minute alterations impede the growth of AVs.
Street Patters Include:
- Grid - a street pattern common in ancient cities and often advocated by New Urbanist planners for facilitating pedestrian access; varaiants of the classic grid include the block grid, curved block grid, and curved grid;
- Loop Streets with minimum and maximum depth standards;
- Cul-de-sacs with maximum length and minimum radius standards of 400-450 feet long and 40-foot turn-around radius
- plaza and hammerhead street style, usually insufficient for vehicular turn-around
regarding street gradients, the minimum gradient in most area is 0.5%. In areas with cold winters, the maximum gradient is 5%; it is 8% for areas with mild winters.
