I understand your concerns, and these need to be implemented well, but let's think about it like this. In Paris, we would have cars that move through with a speed limit of 30km/h. At this rate, we can move 1 car 0.5km/min. Add in traffic lights and traffic, and this gets reduced to our average: ~ 10km/h, or 0.16km/min. So we can do a few things:
1. We can reduce the stop/start time from traffic lights and traffic. If a car could continually move through at 10km/h, it would get to the end point just as quickly as a car that moves at 30km/h with the stop/go of traffic lights. 10km/h seems slow, but the end throughput is just as a high as before, and the rate of accidents drops over 50%, with the rate of accident mortality dropping over 95%. This is a HUGE win: massively decreasing road accidents, while maintaining average time from start to finish for driving.
2. Decreasing overall speed of traffic encourages alternative forms of transportation because it makes them safer. One of the biggest reasons cited from non-bikers is a fear of getting hit by cars. You can either add more bike lanes, or make the routes they already take slower (and thereby safer). This can have a net positive impact on a zone, with zero traffic flow impact.
3. We have seen net movement flow actually INCREASE by reducing traffic speed, and here's how. First, imagine 100 cars moving at 10km/h, or taking about 6.5 minutes to go 1km. An average car length at 4 meters + space between, you end up with ~ 10 meters per car including gaps. 100 cars = 1,000 meters, or 1km. If you take 2 lanes, that means you get 200 cars per km. In Europe the average number of persons per car is 1.5 (in the us, this is closer to 1.1). Let's take our higher number, and we see that we move 300 people in the 6 minutes for a 2 lane road. We also see an average of 3 bikes / minute in Paris (not high for Europe), or 18 people on bikes. Now let's take out one lane, so we are moving 150 people in cars per 6 minutes. Since we consistently see biking multiply by 10x when we provide dedicated space for them, we immediately see ~ 30 bikes per minute, or 180 people on bikes per the 6 minutes. Our net human flow has now gone from 300 people in cars to 150 people in cars + 180 people on bikes, or 330 people total.
We see this time and time again. If the goal is to increase the total number of *cars* moved, by all means, making bigger, faster roads is the way to do that. But if the goal is to increase total number of *people* moved, we see slight increases in car congestion yields such incredible benefits in pedestrian and bike throughput that the ultimate gains far outweigh the increase in congestion. As an added bonus, the slight increase in congestion creates stronger demand for alternatives over time, thus increasing our overall capacity and decreasing congestion. When no alternatives are provided, congestion has nowhere to go.
Un ilustrativo video que muestra la diversidad de acciones que pueden tomarse con el objeto de hacer las calles mas seguras, mas amigables a peatones y ciclistas, calles completas y compartidas que atienden las necesidades de todos los actores urbanos, subordinando a los mas perjudiciales y estorbosos.