Case Study:
Montgomery County, MD

Case Study:

Montgomery County, Maryland

Methodology

Computation

All of the measures were computed based on inputs and outputs of the county's travel model. Montgomery County's model is a P.M. peak-hour model; subsequently the measure calculations use P.M. peak-hour trip tables, where the work end of the trip constitutes the origin i, and the home end the destination j. Data required for the destination accessibility measures include households and employment by zone as well as zone-to-zone travel times by mode. The methods for computing the destination accessibility measures are outlined below in Box 1 (households accessible from employment centers) and Box 2 (jobs accessible from households). The accessibility measures are calculated either county-wide, or for multiple-TAZ activity centers.

Box 1
Calculation of "Households Accessible from Employment Centers" Index

Accessibility from employment center i is computed as:

HA_i = where T_ij < 45 minutes

Where:

i is the TAZs comprising activity center k;

HA_i is the housing accessibility of zone i;

HH_i is the sum of households in all zones that are within 45 minutes of zone i;

Jobs_i is the number of jobs in zone i.

Box 2
Calculation of "Jobs Accessible from Households" Index

JA_j = where T_ij <45 minutes

Where:

j is the TAZs comprising activity center k;,

JA_j, the job accessibility of zone j, is the sum of jobs in all zones that are within 45 minutes of zone j;

JA_k is the job accessibility of employment center k;

HH_j is the number of households in zone j.

The average number of accessible households can be computed and compared for each alternative 2020 network.

The accessibility indices can be computed separately for auto and transit modes, based on the 2020 auto and transit networks. The indices can also be calculated only for "transit" activity centers, to provide a measure of transit accessibility.

In addition, this calculation can be performed to determine "households accessible from jobs" by reversing "jobs" and "households" in the above procedure. It is likely that these measures will closely track each other.

As an example (Table 1), the county estimated the difference in the activity center to households accessibility index between the base 2020 network and recommended plan (for the transit network) to be 66,870. The county interprets this result to mean:

"If you work in an activity center in Montgomery County, and the Recommended Plan is implemented, it will increase the number of houses you can reach from your job site in 45 minutes by transit by nearly 67,000."

Table 1.
Activity Center to Household Accessibility Index (by Transit) for 2020 Recommended Network

Accessible Households
Activity Center	Jobs	2020 Base	2020 Rec. Network	Absolute Change	Percent Change	Absolute Change x Jobs
Bethesda	44,804	299,065	350,945	51,880	17%	2,324,431,520
Clarksburg	3,222	10,443	46,435	35,992	345%	115,966,224
Germantown	20,228	25,418	50,358	34,940	98%	504,486,320
Etc.
Total	232,248					15,529,823,618
Activity Center Weighted Average Increase = Total (Absolute Change x Jobs) / Total Jobs						66,870

Source: Maryland-National Capital Park & Planning Commission (1999).

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