Graph for travel time, auto vs. bus, preliminary



Please note the values shown here for the bus are estimates.

For both the auto and bus lines above, a trip of 0 miles does not exist. I simply solved for 0 miles which is also called the "x intercept"

Both lines above are generated using the y=mx+b type of linear equation.

For the auto, the travel time y in minutes for x miles is:

y = travel time in minutes
x = trip distance in great circle miles (or "as the bird flies").

m = 1.41 minutes/mile
b = 12.09 minutes per trip

The automobile coefficients are based on 46 vehicle trips measured with a GPS (global positioning system).

For the bus, I used a set of 6 trips using travel times calculated using the samtrans.org web site.


m = 12.91 minutes/mile
b = -14.96 minutes per trip

===

The next step is to compute and compare total percieved cost of a given bus trip with a car trip.

The "perceived total cost" depends on a very simple economics generalization that I found in a 1970 vintage economic analysis of the decision to extend the Victoria Subway system in London.

The economists generalized the value of a person's time at 1/3 that person's hourly wage. If you earn $15/hr at work, then the approximate value of your personal time spent doing your daily journeys is $5/hr.

(Footnote to original research paper needed).

So the next step in my freight carts on buses analysis is to compare the total perceived cost of riding a bus with the total perceived cost of driving a car, for a set of realistic distances using travel time data that reflects actual experience.

I am particularly interested in setting up some data input forms so the reader of this blog can change the input values. For instance, just how high does gasoline have to go in price before I switch to riding the bus to work?

I know $4 a gallon gas got me concerned enough to start this blog.

When I reach the point where the cost comparisons have plausible supporting data then it will be possible to explore what economic or physical changes will make freight carts on buses an attractive transportation mode for American society.

Linear equation for travel time by a car

I have finally finished re-creating my linear regression travel time analysis.

Based on measuring 46 car trips in my 1993 Dodge Van, with many trips starting from my El Granada neighborhood, here is a formula for the time required to drive places in a car:

  • Given the distance in miles measured as a bird would fly (great circle miles)

  • Time = 1.41 minutes per mile + 12.09 minutes

Details of the data:
  • The measurement covered 46 vehicle trips.
  • Average distance was 7.90 miles.
  • Average trip time was 23.2 minutes.
  • Average speed for all trips was 17.72 miles per hour.
Another way of looking at the transportation formula is:

1.41 minutes per mile implies 42.55 miles per hour... (take the reciprocal and multiply by 60 to convert miles per minute to miles per hour).

That is pretty reasonable as my neighborhood has mostly 45 and 50 mile per hour speed limits on Highway 1.

The travel time formula is an approximate empirical constant that reflects the performance of a car as a transportation device.

So a car trip consists of "About 12 minutes plus you move toward the destination at 42.55 miles per hour."