20 is Plenty and Other Crazy Thoughts

Earlier this year in an online forum on Reddit, I laid out my quick list about what the top transportation needs are for Roswell.  My list in no order of importance and applicable to most cities was as follows:

  • Bring MARTA Rail to North Fulton
  • Increase Street Connectivity
  • Remove Reversible Lanes
  • Build More Roundabouts
  • Drop speed on ALL residential streets to 20 mph
  • Build the Roswell Loop

Most of my suggestions focused on increasing transportation options and improving safety.  Interestingly, the one that got questioned was the point about dropping speeds on residential streets to 20 mph.  When I indicated that my rationale was for safety reasons, one commenter insinuated that this isn’t necessary because we don’t have a pedestrian death problem in our residential areas.  I agree and we should feel fortunate for that.  However, I think many a homeowner can point to multiple occasions where they have encountered drivers speeding recklessly on neighborhood streets.

Speed has a logarithmically negative effect on survival rates for pedestrians involved in collisions with cars.  A 10% increase in vehicle speed increases pedestrian fatality risk by 40-45%.  Data shows that when a pedestrian is hit by a car traveling at 20mph, they have a 95% chance of survival.  However, as the speed increases, the survival rate plummets.  When a car is traveling 40 mph, the pedestrian survival rate drops to just 15%.  This is just plain physics.  Doubling speed results in the required stopping distance quadrupling and the kinetic energy absorbed at impact is also fourfold.   We may not have a death problem here in North Fulton but nationwide, more than 30,000 people are killed in car crashes annually and an increasing percentage of those are pedestrians.  Ten times that number are seriously injured every year.  The costs to society are staggering but we accept it as a necessary evil to support our auto-dependence.

Slowing down to 20 mph is a radical idea that would increase safety in our communities for pedestrians and cyclists alike.  That said, simply lowering speed limits isn’t a panacea.  Drivers generally drive at the speed they feel safe regardless of the posted speed limit.  This comfort zone, the speed that feels safe, can also be called the design speed or the speed at which the road was designed to be safely navigable.  The philosophy of wider, safer, faster holds true here.  The wider the road is, the safer it feels at higher speeds.  This counterintuitively increases speeds which conversely decreases safety for everyone involved.  

We’ve all lived in or driven through residential subdivisions with streets wide enough to fit parked cars on each curb and two active lanes.  The problem with this is that the streets in suburban residential areas are rarely every fully lined with parked cars.  The end result of this is a dangerous design with wide expanses of asphalt that encourage teenagers to test their limits and rushed commuters to push the gas.  This would happen in this environment regardless of whether the posted limit was 30 mph, 25 mph or 20 mph.  It’s just not conducive to a slow drive.

That said, a 20 is Plenty campaign such as those that are having great success across Europe and the UK would be a bold step to create safer and more walkable cities.  The movement is slowly making its way to the US and is now under consideration in several cities and towns in the northeast.  New York City is considering it and some people have even gone around town putting up their own signs on light posts.  There is also research indicating that slower street speeds are linked to more social connections, a stronger sense of community, higher property values and increased walking and biking.  

It almost sounds like a no brainier.  So, this is how I would propose phasing in a 20 is Plenty campaign: 

  • Step 1 (year 1-3).. Give all neighborhoods and subdivisions the option to adopt a 20 mph limit.
  • Step 2 (year 4).. Adopt on all roads that have residential as  > 50% of their frontage and on any road that fronts a school or park for a quarter mile in each direction.
  • Step 3 (year 1-10 and beyond).. Re-engineer streets over time to narrow lanes and install street calming devices that would encourage slower speeds.

The next time you’re driving through a neighborhood remember that 20 is Plenty.