Lesson 7 - Transportation Flashcards
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).
Defining trip generation rates
A special type of survey, known as an Origin-Destination Survey, is used. This survey requires that roadblocks be set up 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. That 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 becomes less and less valid with age. When local surveys are unavailable due to time or monetary constraints, published rates are used to derive estimates. One must keep in mind that trip generation models, tables, and surveys all have their own sources of error and are good for only providing estimates.
Trip end
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
is a rather simple tool that attempts to quantify the rather complex trip generation relationships. It provides trip estimates based directly on the proportional attractiveness of the zone and inversely proportional to the trip length. The Mid-Ohio Regional Planning Commission (MORPC) link provides some useful transportation planning related information and examples.
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 their cars, various forms of mass transit, bicycles, or walking, we are able to estimate how many vehicles need to get from one place to another. Some important definitions to keep in mind include:
AADT (Average Annual Daily Traffic) is 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 is the peak hour volumes during different seasons;
Design Hour Volume (DHV) is 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 by the hour. It basically gives us the information on the routes that will be used. Peak volumes can then be compared with DHV to see which, if any, roadways are going to experience traffic over their design capacity; i.e., where the congestion is going to be.
Vehicle Miles Traveled (VMT)
is a measure of vehicular mobility obtained from travel inventories. Vehicle miles of travel 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 further apart and travel times are becoming longer.
The City of St. Louis, provides an example of the VMT. Their Key Indicators of Community Change Report provides an example of how VMT is used.
The Bureau of Transportation Statistics tracks VMT at a national level. Take a look at the number of highway vehicle miles traveled.
Transportation planning uses the information on future land uses to project future volumes and traffic patterns that can then be incorporated into current and near-future transportation decisions. All of this information is crucial in deciding which roadways need to be expanded, and in determining how land use controls need to be modified to mitigate traffic flow.
Road design
focuses on everything from the nature of street hierarchy to actual design guidelines for local streets. While local or residential streets are designed to serve only local land uses, collectors are designed to serve some land uses directly while collecting traffic from local streets and funneling it to major roads and arterials. The main purpose of major roads or arterials is to further move traffic and allow access to freeways or other streets. Freeways, which have limited access, are responsible for moving the largest volume of traffic at the fastest speed.
Ideal road cross section
The cross section of an “ideal” road would have ten foot travel lanes, eight foot parking lanes, and a curb and planting strip. However, the resulting ROW of 56 feet is much wider than most local streets.
Roadway standards origin
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. Through frequency and repetition, rather than by proven effectiveness, these guidelines evolved into our modern standards. In most cases, these standards are too wide for most local streets, and are most advantageous if there is a lot of traffic and no off-street parking.
Typical local street standards
include:
500 feet maximum tangents;
Use of stop signs or speed bumps to reduce vehicle’s speed;
150 feet between intersections;
Clear sight distances of 75 feet.
Basic road patter
As street classifications change, so do the standards. For example, the tangent and curve radii should be higher on collector streets because clearer sight distance is needed at higher speeds.
The most basic road pattern is the grid, which was invented by the ancient Greeks. Its advantages include the ease of laying out streets and lot lines and the ease of navigation (way finding). Disadvantages of the grid include:
Maximum number of 4-way intersections, which can create conflicts;
Tangent standards can not always be met;
Grids result in the maximum pavement and utility line costs;
Streets can be used as parallel short-cuts during rush hours;
Grids can be very monotonous and boring.
Variants of the classic grid include the block grid, curved block grid, and curved grid.
Other street patterns include:
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, which do not provide enough turn around.
Regarding street gradients, the minimum gradient in most areas is 0.5%. In areas with cold winters the maximum gradient is 5%; it is 8% for areas with mild winters.
Highway Capacity Manual
The Highway Capacity Manual, published by the Transportation Research Board, provides concepts, guidelines, and procedures for computing highway capacity and quality of service based on road type.
The Highway Capacity Manual is published by the Transportation Research Board.
