Posts Tagged traffic
Diverging diamond interchanges speed up traffic, but they feel weird
Posted by mark in engineering on July 3, 2025
Sarasota County, like all coastal areas in South Florida, gets jammed up by ‘snowbirds’ like my wife and I, who migrate there every winter to escape Minnesota’s painfully cold weather.
To alleviate traffic at its heaviest near the city of Sarasota, the Florida Department of Transportation (FDOT) converted the junction of Interstate-75 at University Parkway to a diverging diamond interchange (DDI)—the largest in the world at the time it was implemented in 2017. This development increased capacity by 33 percent to 80,000 vehicles per day and reduced the intersection delays by 50 percent. These improvements stem from a large reduction—over 50 percent—in the conflict points.
Soon the I-75 at Fruitville Road interchange will also be converted to a DDI by FDOT. Hundreds more are either already built or under construction across the USA. If you haven’t experienced a DDI, get ready for being disconcerted from the temporary reversal of lanes.
I hate DDI’s so much, that when forced to drive down University to pick visitors up at the Sarasota Bradenton Internation Airport, I exit I75 at Fruitville Road. But this resistance is fruitless (pun intended) due to the upcoming project by FDOT.
I also dislike roundabouts when traffic gets to the point where it becomes very difficult to break in on the flow. Sarasota County features a great number of traffic circles like this. The mix of slow senior snowbirds and aggressive Florida men aggravates the free-for-all.
All this makes route planning and timing essential when driving during Florida’s peak season. Fortunately, I enjoy challenges like this that involve multiple constraints. As the guru Sivananda Saraswati said, “the harder the struggle, the more glorious the triumph.”
Twenty declared plenty: How slow will we go?
As reported earlier this week by the Center of the American Experiment, motorists face a new 20 mph speed limit in Minneapolis and St. Paul. City authorities figure on a significant reduction in neighborhood traffic fatalities, based on the statistic that a person hit at 35 mph is three times as likely to die as someone hit at 25 mph (they are reducing limits another 5 mph to 20 mph out of an abundance of caution, presumably). Prior to a new law that came into effect a year ago, the Minnesota Department of Transportation (MnDOT) set speed limits based on engineering and traffic studies. But now cities need not involve MnDOT when setting traffic laws for residential streets.*
The lowering of speed limits in the Twin Cities follows a trend in USA metro areas from coast to coast as evidenced by this Seattle Department of Transportation post last December (check out the animated graphic showing how a person’s chance of surviving being hit by a car decreases drastically with faster speeds).
My thoughts:
- If 20 mph on residential streets got enforced, that would be a relief for those like me with young children at home (grandchildren in my case). However, I doubt this will happen, especially with cutbacks in police after the troubles in Minneapolis earlier this year. The lower limits will only work with plentiful speed bumps (more appropriately known as “sleeping policemen” in UK).
- Being an engineer, I worry about taking experts on traffic safety out of the loop in favor of politicians making sweeping edicts with no regard for varying factors for individual streets.
- What are the economic trade-offs of the added time needed to travel at slower speeds versus the increased safety? Is 20 mph optimal?
“Typically, drivers travel 8 to 10 mph above the posted speed limit with a perception that the posted speed limit is a minimum, not a maximum [and] when the posted speed limit is reduced, drivers do not obey the new limit or even pay attention to it unless there is significant enforcement.”
Research Brief: Review of Current Practices for Setting Posted Speed Limits, April 2019, AAA Foundation for Traffic Safety.
One thing for sure, I find it excruciating to drive at 20 mph for any distance. The seems to slow to me.
*Focus on New Laws: Cities Authorized to Set Certain Speed Limits, July 22, 2019, League of Minnesota Cities.