Maybe your worst nightmare...

Ah, Paris without the traffic, strolling down the Champs Elysee and walking across the Place de la Concorde.
What could be better? I see no disadvantages.

Why is this a nightmare sounds like the best time to see the city no noise from traffic no risk of being knocked down. More places acroos the globe should follow suit.

I think I'll jaunt over from Arras that day. I want to be able to walk without the pollution. Is it a nightmare because the 16e will be busy?

It could be a nightmare for those arriving at or going to CDG/ORY who feel they need to take a taxi or car service instead of the very convenient rail service between Paris and its 2 major airports. But even in these cases, according to the OP, taxis are exempt so 'nightmare' averted. At 30 km/h, they'll travel at the rapidity of snails.

We were in Paris a few years ago when they did this - it was interesting watching some guy try and 'windskate' down the street (with a skateboard and a sail). Fun times!

Guess I'll go with my original plan and spend the day in Budapest - the Paris of the East????

This sounds delightful! ---That is unless you absolutely need your car that day!

So . . . no horses allowed? And merci, chexbres, I didn't realize the name for that device was a trottinette.

They're popular in Lyon, but it's always been strange to these American eyes to see an adult pushing themselves along on what looks like a child's push-scooter.

You pose an excellent question Cyn! Presumably horses are environmentally friendly, no?

Hi Kim - although horses might be a nightmare (or night-stallion, or night-gelding) for the street cleaning crew! ;-p

Fantastic ! That is a real incentive to get back to Paris tout de suite.

Interesting if this becomes and annual event. I'll be in Paris the first Sunday of October next year. Would be a sight / site to see.

Thanks so much for the notice! I'll be in Paris that day and will include some walking!

This is in fact the third year they've done this -- it's already annual (Sept. 2015, Sept. 2016). They have decided to close off MORE streets this year than last year -- each year organizers have closed off more of the city to regular traffic, arguing that the previous year's mishmash of designated and non-designated areas was more confusing than it was worth and that it would be straighter and simpler for people to understand to go "whole hog," so to say.

As a resident, I tend to be a bit of a grump about it too, but oh well.

Here's an article prior to last year's car-free day:
https://www.forbes.com/sites/ceciliarodriguez/2016/08/22/paris-will-stop-all-cars-literally-for-a-day-in-september-once-again/#45e035c25484

You can see here a map of the areas designated car-free in 2015 (even if you don't read French, you can easily get the idea of some more central zones being designated as car-free):
http://www.leparisien.fr/paris-75/paris-75005/journee-sans-voiture-a-paris-la-carte-des-quartiers-qui-seront-interdits-29-07-2015-4978025.php

And here's a map showing 2016 (more of the central zone covered by car-free designations):
http://www.liberation.fr/france/2016/09/24/paris-la-journee-sans-voiture-a-quoi-ca-sert_1507617

And now 2017's map (for October 1):
https://www.paris.fr/journeesansmavoiture

In fact, the 16th isn't excluded. A tiny bit -- the Bois de Boulogne, and a couple of access points thereto -- is excluded.

Hidalgo is quite serious about her anti-cars initiatives. This is all of a piece with other efforts.

Mobility in large European cities includes multi-modal options. It is not defined narrowly by cars (and never has been), so how is this a "nightmare", much less a "worst nightmare"? It sounds like a great idea to me...giving people their streets back minus all the noise, pollution, hassle, etc. Tourists will just need to adapt, it's not the end of the world (nor does the world revolve around their comfort). By the way, we have a Car Free Day here too in the Washington DC Region but it's voluntary and relies on coaxing and marginal prizes by project sponsors...so it's very limited in its effect. Even car-centered Los Angeles has CicLAvia once a year, which is very popular.

You still can use Metro, bus, taxi, bicycle, so what's the big deal? I think I would like it. Tourists especially should not mind because they very rarely use their (rental) car for transportation around Paris.

It can be a nightmare to a lot of tourists, who have difficulty getting around, are elderly, infirm and not used to walking much.
Everyone else who is young and healthy can cheer them on, as they go by with their walkers and wheelchairs...

The police want to know why you are using your car or a taxi to transport sick people. That's why you need a "justification de domicile".

The pollution - or lack of it - is negligible. It's a publicity stunt by the Mayor's Office.

Granted the small portions of the 16th don't really matter, because most tourists do not go there.
I would allow a lot of time to come and go from CDG, Roissy or any major train stations.

" It's a publicity stunt by the Mayor's Office."

Well okay, now we're getting somewhere. Do a significant number of Paris residents feel this way? Is the Mayor up for reelection or something-or trying to gain points with certain locals, or tourists? What does this do for the Mayor's office?

Pollution not a problem?
http://www.airparif.asso.fr/_pdf/publications/Air_quality_Paris_region_2016_PR.pdf
https://www.thelocal.fr/20170216/air-pollution-france-gets-slapped-with-final-warning-from-eu
https://www.forbes.com/sites/ceciliarodriguez/2016/12/12/paris-cant-breath-worst-pollution-in-a-decade-has-city-scrambling-for-solutions/#29a815191c2f

I have to say, as I watch the coverage of the millions of people who have suffered and lost so much in the recent hurricanes and earthquakes, I have trouble thinking of a traffic congestion free day in Paris as a nightmare at all, let alone "your worst nightmare".

Actually, Chamonix, à boxed in valley has the most pollution alerts. But don't tell the tourists.

And a big like to Larry. Add Mexico City to his list. Those are nightmares.

Routier is a truck driver. Expect traffic jams everywhere.

I was in Paris during such a day last year.

Was really nice actually ; however last year the buses did not run yet didn't understand the rules until late morning so were selling tickets and operating as normal in the early morning.
We were on one of the double decker busses when all of a sudden it stopped, the driver talked on the phone for 30 minutes to his dispatchers while we sat not knowing what was happening and then they told everyone to leave the bus and no more busses would be moving that day.

People were quite upset, since passengers paid for the daily pass and they were not giving any refunds.
We had bought the combo bus and boat pass so were able to ride the boats so was not a major inconvenience for us and having the streets clear was great!

Seems like they are changing that this year?
OpenBus and BigBus will run at 30 km/h, and will run their regular routes.

My worst nightmare in Paris is all of their public transportation going on strike.

Streets blocked to vehicular traffic should be an opportunity for those of you who live in a car-centric region to TRY the métro if you don't care to walk.

I hear the beach at Trouville or Berck-sur-mer are nice, this time of year...

No fear of all mass transportation going on strike in Paris. There's always something running.

A lot of people who own cars don't understand why people who live in Paris are so tired all the time.
It takes a lot of time to go to the market(s), walk home, haul your stuff up the stairs - then go to the pharmacy, because you forgot the Doliprane.

A lot of people who own cars don't understand why people who live in Paris are so tired all the time.
It takes a lot of time to go to the market(s), walk home, haul your stuff up the stairs - then go to the pharmacy, because you forgot the Doliprane.

Most everything is closed on Sunday, so it's a moot point.

A lot of people who own cars don't understand why people who live in
Paris are so tired all the time.

It sounds like it has more to do with one's age and physical health whether one feels tired all the time. Millions of people in Manhattan and other large cities don't own cars and they manage to get on with their lives regardless. Driving doesn't necessarily save time in environments that are designed around other modes of transport that are faster and cheaper.

Have you ever lived in Manhattan?

Have you ever seen an elderly man or woman try to pull a caddie full of groceries up 4 flights of stairs? Or a young mother with twins and a stroller in the same situation?
They are troupers, that's for sure.
Not everyone is healthy, and Paris has a rapidly aging population.

Of course, all the supermarkets deliver groceries - whatever floor you're on, even without an elevator.