Average L.A. Freeway Traffic Versus Time and Day of Week

Overview
Tables of maximum congestion by freeway
Plots of the percentage of sensors indicating congestion versus time of day, by freeway

• When are the worst times to drive on Los Angeles freeways?
• How should I plan my travel to give me the best chance of not encountering traffic delays?
• What is the penalty in time to drive the freeways at the worst times?

These are questions of more than academic interest to many people. I've wondered about them ever since I moved to Pasadena, and now, thanks to the wonderful service provided by Caltrans and Maxwell Technologies, I have been able to find out.

Maxwell made available the Los Angeles Traffic Report. I used their service to download Current Congestion Locations in L.A. County for 19 days from 24 January to 25 February 1997. Even though it is probably easy to write a script to automatically download such data, I did it manually, since it is relatively easy to do this without much effort.

(The L.A. Traffic Report page has since moved to SmarTraveler. The speed map is reached by clicking "click here for the SmarTraveler real-time traffic map", the second link from the upper left, which is a "blinking" link.)

Of those days, the weather was good and conditions "normal" except for rain on Saturday, 25 January and rain on Superbowl Sunday, 26 January. Also, my sampling of the morning commute is not terribly complete, since I am not a morning person and had to make a special effort to get any samples at all before 9 or 10 a.m.

There are 1,383 sensors embedded in the freeways and reported on Maxwell's website. Maxwell's website color-codes the current state of each sensor as follows:

For these initial calculations, I have defined bad congestion by a sensor that is red, yellow or blue, and then calculated the percentage of such sensors at any given time, subtracting those with sensor errors from the total possible number of sensors.

To get an overall impression of L.A. traffic, I used all sensors on all freeways. Five remarkable facts stand out:

• The average L.A. traffic congestion pattern is almost like clockwork, following the same pattern day after day.
• 25-32% of all sensors in L.A. county indicate bad congestion during the morning rush hour and the afternoon rush hour from Tuesday through Friday!
• 5-10% of all sensors indicate bad congestion at all times of all days, except Sundays and possibly between midnight and 6 a.m. (I collected no data then).
• Fridays are by far the worst traffic day for the evening commute of the weekdays, directly contradicting the "Friday Light" claims made by some. (There is some weak evidence that the Friday morning commute is a bit lighter than the other days.)
• In contrast, the Monday afternoon rush hour congestion is always the lowest of the week.

Further comments and caveats:

• The Saturday with rain had significantly more congestion than the other Saturdays and Sundays that I examined.
• Even though the average L.A. traffic congestion pattern is quite repeatable, the actual current traffic on any individual freeway is much more variable. That is what makes Maxwell's website very valuable. See 210 East Freeway from Hill to Sunflower for two examples of variability caused presumably by accidents, one on Wednesday noon and one on Monday 11 p.m.
• When 30% of all 1,383 sensors indicate slow traffic, 100% of the sensors in many areas of L.A. indicate essentially stopped traffic.
• The conclusions I have drawn above strictly apply only to the period that I analyzed, 24 January to 25 February 1997. I'll analyze some summer traffic later to see if any changes occur then. Holiday weekends probably generally have more traffic: Caltrans reports that motorists typically travel 400 million miles each day on California's freeway and highway system, and 450 million miles each day each of the four days of Labor Day weekend, Friday through Monday. (LAT 9/3/99, B1)

I will add more freeways at the rate of 1-2 per week until they are all present.

Sorted by maximum congestion

Sorted by worst total time with over 15% of sensors indicating bad congestion, for afternoon rush hour

I have intentionally left off the titles of the axes of the following plots, in order to make more room for the data. Every plot is the same: the x axis (horizontal axis) gives the time of day ranging from 6 a.m. to midnight. The y axis (vertical axis) gives the fraction of sensors indicating congestion, defined as the number of sensors coded red, yellow or blue by Maxwell, divided by the total number of working sensors reported for the freeway being plotted. Thus, for example, the maximum value of 0.33 that is reached for all sensors on all freeways means that 33% of all the operational sensors report congestion. The maximum value of 1.0 for individual freeways means that every single operational sensor on that freeway reports congestion. Within each freeway being plotted, the scales of the graphs are identical, so that one can see at a glance which time periods are worst and how bad they are compared to the rest of the times.

For the average L.A. traffic on all freeways, I present separately plots by day of the week, as well as a plot giving all weekdays, and one giving just Tuesday through Thursday, since the traffic on Tuesday, Wednesday and Thursday is very similar. For individual freeways, I usually present just the necessary plots to convey what is happening on that freeway versus day of the week. When more than one day is on a plot, I have color-coded each day separately, with the colors for monday through friday following the progression through the rainbow. For some reason, the export of these plots from quattro pro didn't always preserve the colors, so the line styles for Monday and Friday are different as well.

All plots can be clicked to give a larger and much more readable full-scale version. They have been reduced by 50% in each axis on the main page for each freeway in order to compare the plots more easily.

I will add more freeways at the rate of 1-2 per week until they are all present.

Available Plots

Now, regarding the final question I asked at the beginning:

What is the penalty in time to drive the freeways at the worst times?

The answer to that will come after I put the rest of the freeways online!

Update log for L.A. traffic and the rest of T. Chester's calculations

Go to Calculations

Copyright © 1997-2000 by Tom Chester. Permission is freely granted to reproduce any or all of this page as long as credit is given to me at this source:
http://la.znet.com/~schester/calculations/traffic/la/index.html
Comments and feedback: Tom Chester
Last update: 4 September 1999 (L.A. Traffic link updated 12 July 2000).