Predicting demand
The bicycling and walking modes have experienced decades of neglect in mainstream transportation planning practices and roadway design. The results are seen in nearly every city and town across the country - communities were built without sidewalks, and roadways have no additional space for bicycling. As cities and towns begin the work of redeveloping their transportation systems to support bicycling and walking, the list of needed improvements far outstrips available dollars. So planners have begun to look for ways to set priorities - one of which is predicting demand.
The question that planners have begun to ask is this: if we build this bikeway (or walkway, etc.), how many people can we expect to use it?
Finding the answer is the fundamental aspect of predicting demand. Transportation planners have been asking (and answering) this question for motor vehicular travel since the late 1960's, when the first travel demand models were developed. By contrast, bicycle and pedestrian researchers are only just beginning to scratch the surface on these topics.
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For bicycles and pedestrians to have a seat around the transportation table, it is important to acknowledge that some level of analysis must also be done for these modes. Transportation planners have a responsibility to ensure that public funds are being spent wisely - in locations where a larger number of people will benefit from new facilities. Finally, there is a growing trend to quantify the air quality benefits (and congestion relief) that can be expected as a result of CMAQ (Congestion Mitigation and Air Quality) projects. For bike and |
pedestrian facilities this means coming up with some way to determine how many auto
trips will be diverted.
While the science of predicting bicycle and pedestrian travel demand has not yet developed to the same level as motor vehicle planning, there are a number of methods that planners have developed over the years to help quantify which locations have higher levels of demand. When planning bicycle and/or pedestrian facilities, it is important to remember that current volumes usually do not reflect demand for two reasons:
1) existing conditions and gaps in the network result in fewer users - potential users are deterred by dangerous conditions.
2) dispersed land uses create trip distances that are perceived as being too far to make on foot or by bicycle.
There are two methods of determining demand for bicycle facilities: the intuitive approach vs. the use of demand forecasting models. The intuitive approach is less time consuming, however it does not yield precise results. This type of planning analysis is also called a "sketch plan." A sketch plan typically focuses on proximity between origins and destinations, since distance is a primary factor in the initial decision to take a walking or bicycling trip. According to the NPTS, the majority of pedestrian trips are 0.4 km (0.25 mi) or less, with 1.6 km (1 mi) generally being the limit that most people are willing to travel on foot. In other words, most people are willing to take a five to ten minute walk at a comfortable pace to reach a destination. The majority of bicycle trips are 4.8 km (3 mi) or less - or about a 15-minute bike ride.
NPTS data also shows that land use patterns and population density have a big impact on trip distance. Higher density communities with mixed land use patterns will have higher levels of walking because destinations are more likely to be located within walking distance of homes and businesses.
For an intuitive (i.e. sketch plan) approach, destinations throughout the study area that would attract bicyclists and pedestrians are shown on a base map. Routes are selected that serve higher concentrations of destination points, or that serve destinations that typically yield high numbers of bicyclists and pedestrians, such as universities, downtown areas, shopping centers, major employment centers (hospitals, business parks, major industries and corporations, etc.), schools, and parks. Route selection and prioritization can be done via graphical representation; the intent is to identify locations that serve multiple destinations and higher population densities (population densities can be obtained from census data). This methodology can be accomplished using a GIS system or it can be done by hand.
Public involvement is
important to the success of the intuitive (sketch plan) method. It
is particularly important to gain input from a wide variety of local
citizens (representing different geographic areas) who represent all
ages and abilities.
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The other method of estimating latent bicycle and pedestrian travel demand is to adjust conventional motor vehicle travel demand theory so that it applies to bicycle and pedestrian travel. By using a gravity model to measure latent bicycle and pedestrian travel demand, the planner can achieve results that are more precise than the intuitive approach. The other advantage to this approach is that it compliments the type of analysis that is typically done for motor vehicle and transit travel simulation. This can be particularly important in |
cases where bicycle improvements are competing for similar funding mechanisms as other modes, since most transportation improvement programs make funding decisions based upon quantifiable results.
Bicycle and pedestrian travel demand modelling can be done on a system-wide basis, or at the corridor level. Further information on more precise bicycle and pedestrian travel demand methods are provided in FHWA Publication No. FHWA-RD-98-166, Guidebook on Methods to Estimate Non-Motorized Travel, 1999.
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