Transportation Geography and Network Science/Mobility

Mobility can be defined as the ability of individuals or goods to move through a transportation network. Mobility encompasses a set of network measurements which quantify the use of a transportation system, but is also used in relation to the ability of individual social, economic, or demographic groups to utilize a transportation system. Mobility definitions are often extremely similar to accessibility, with some agencies using the terms interchangeably. To avoid this confusion, mobility will be treated as "ease of movement" whereas accessibility refers to "ease of reaching a destination".

Mobility Measurements

A wide variety of measures are used to quantify the mobility of a network. These include:

• Person-miles traveled
• Person-trips
• Ton-miles traveled (for goods)
• Average travel speed (for all modes)
• Travel time reliability
• Transit/cycling Level of Service

The Florida Department of Transportation (FDOT) developed a set of mobility performance measures for all modes: Auto/Truck, Transit, Pedestrian, Bicycle, Aviation, Rail, and Sea. These measures are broken into four categories to capture the:

• Quantity of travel performed
  • Vehicle-Miles
  • Person-Miles
  • Trips
  • Tonnage
• Quality of travel performed
  • % Travel at LOS Criteria
  • % Miles at LOS Criteria
  • Travel Time Reliability
  • Travel Time Variability
  • Vehicle-Delay
  • Person-Delay
  • Travel Speed
• Accessibility provided by the transportation system, and
  • % Sidewalk Coverage (Pedestrian)
  • % Bike/Shoulder Coverage (Bicycle)
• Utilization of a facility or service
  • % Miles Severely Congested
  • % Travel Severely Congested
  • Hours Severely Congested
  • Vehicles Per Lane-Mile

The aggregate measures indicated above, such as total auto-based vehicle miles traveled or total tonnage by rail, are found simply as sums based on direct counts or through surveys/sampling techniques. Travel time reliability and variability are defined as:

• $TravelTimeReliability={\sum {VMT|TravelSpeed\geq SpeedLimit} \over \sum {VMT}}*100$

• $TravelTimeIndex_{95}={TravelTime_{95thpercentile} \over TravelTime_{freeflow}}*100$

Thus, travel time reliability measures the percent vehicle-miles traveled at freeflow conditions with a larger fraction indicating better performance. Travel Time Variability, given as the Travel Time Index, measures the 95th percentile travel time and compares it to freeflow travel time. Each of these measures is used for a given corridor or origin-destination pair. Comparisons of travel time between two disparate corridors cannot be used within the same calculation.

Within the Oregon Department of Transportation, significant effort has been focused on maintaining mobility during construction season. To this end, major reconstruction areas are examined based on "critical route pairs". Paired routes are alternatives; when one of the pair is under heavy restriction due to construction, the alternative route must maintain maximum operation in order to handle increased load. Additionally, freight carriers must meet additional permitting requirements targeted at minimizing delays.

Mobility of Groups

Mobility is has been used to describe the ability of social, economic, or demographic groups to access and utilize transportation systems. Most frequently, disabled and elderly groups are identified as requiring special consideration for mobility. In the Twin Cities, Metro Mobility is a supplementary transit system targeting individuals who:

• Are physically unable to get to regular fixed-route buses
• Are unable to navigate regular fixed-route bus systems once on board, or
• Are unable to board or exit the bus at some locations

Similar systems, referred to as mobility services or para-transit services, are offered by many transit agencies nationwide.