What is the capacity of I-94?

I recently wrote about the Capacity of the Green Line, demonstrating a huge amount of underutilization under both reasonable and unreasonable assumptions.

I thought I should do the same for I-94 in Minneapolis and St. Paul. Of course a freeway and an LRT are different things, so the kinds of assumptions and the available data differ.

  • 1800 vehicles per hour per lane (today, 2 second headways) vs. (with automation, 1 second headways) 3600 vehicles per hour per lane.
  • 4  (12-ft) lanes today vs. 8 (narrower, 6-ft) lanes with automation
  • 4 passengers per vehicles max vs. 1.5 passengers per vehicle today.
  • 4 lanes in each direction, 2 directions

We also need to figure out the average length of trip, which is a bit less obvious.

Current two-way Average Daily Traffic on I-94 on the peak section (near Riverside) is about 164,000: The number of entering trips (based on sum of Eastbound entering vehicles at on-ramps between and including Hennepin Avenue (Minneapolis) and Dale Street (St. Paul) is 161,000 [A distance of about 9 miles or 14 km]). The average number of Eastbound through trips in the Lowry Tunnel is about 87,000.

Entering flows on I-94 from Lowry Tunnel to Dale Street (and final flows in St. Paul)
Entering flows on I-94 from Lowry Tunnel to Dale Street (and final flows in St. Paul)

Average length is vehicle miles traveled divided by number of trips. If we have an average flow of 80,000 vehicles in each direction for 9 miles, this is 720,000 vehicle miles traveled in each direction for this long section. The number of entering trips from Lowry Tunnel to Dale Street (eastbound) (inclusive) is about 240,000. This implies the average vehicle which uses I-94 between the cities uses the facility for 3 miles (5 km). Obviously this is an approximation, but it is probably not too far off. There are 16 entrances over 9 interchanges over this span. Some have parts a, b, and c, and in the sequence (Exit 231 to Exit 240, inclusive). Note Exit 232 (LaSalle?) is missing. So we will go with 9 exits, or 1 mile between exits, and an average trip of 3 exits). The table shows some capacities. This assumes 4 lanes throughout, which is also not strictly true.

A C D
Automated and Unconstrained Today’s Flow  Constrained
Hours

24

24

10

Vehicles per Hour per Lane

3600

833

750

Vehicles per Day per Lane

86400

20000

7500

Person Capacity per Vehicle

4

1.5

1.5

Numbered Exits

9

9

9

Lanes

8

4

4

Average Trip Length (in Exits)

1

3

3

Directions

2

2

2

DAILY PERSON CAPACITY

49,766,400

720,000

270,000

In a world of automated vehicles, if everyone made short (1 exit) trips, in fully loaded (4 persons) per car, fully utilized over 24 hours per day, I-94 could carry about 50 million people per day over this stretch.  In contrast, today it carries about 720,000 people. [If we evaluate the Green Line as 2 lanes and I-94 traffic per lane, I-94 produces more person trips per lane than the Green Line]. If we were to constrain it further, so it only operated 10 hours per day (recognizing people travel only during certain hours), it would carry fewer people than it does. This is just a thought experiment to get some magnitudes. But clearly we have a lot of potential capacity in the years ahead as automated vehicle technology becomes mainstream, if we manage our roadspace and our vehicles more carefully. This argues against capacity expansion.

We conclude that Car Culture remains dominant. The number of people using I-94 on a given weekday is about 20 times larger than the number of people using the Green Line.

A $1 billion transit investment is rounding error for the change in traffic count on the parallel highway (comparing entering vehicles between Lowry and Dale for October 2014 (244,103) and October 2013 (244,712) – average weekday traffic ). Overall trends are mostly flat, with a slight uptick in vehicle miles traveled in 2014 nationally, but the core cities (Minneapolis, St. Paul) added population faster than they added jobs last year (I believe, I don’t think the data is solid on this), so average trip lengths should otherwise have dropped slightly in the cities, independent of national trends.

Also a caution, University Avenue may be different, but the data on changes in traffic counts before and after the Green Line opening is not publicly available as far as I know.

Transit investments like the Green Line LRT serve transit users, highway investments like I-94 serve highway users, they are completely different markets and, this data suggests that at this point in history,  barely substitutes. Congestion reduction should not be a selling point for transit investments, just as reducing crowding on trains or buses is not a valid selling point of highways.

Acknowledgment:

The data comes from the Minnesota Traffic Observatory and the Minnesota Department of  Transportation from their DataExtract program. All analysis is the responsibility of the author.

Cross-posted at streets.mn