Monday, April 23, 2012

Streets Where Traffic Naturally Goes Slow

By Jeff Watton


One method of traffic calming involves the use of horizontal curves in the road.  Sharper curves require drivers to significantly reduce speed in order to safely execute the turn.  This traffic calming device functions in a similar manner to speed humps.  While both horizontal curves and speed humps can reduce motorist speed at a point on a road, the repeated use of these tools within close proximity to one-another is necessary in maintaining reduced speed throughout.

Although these devices are effective traffic calming measures, they can be dangerous for unsuspecting motorists.  For motorists that fail to slow down, speed humps can cause the driver to damage his/her vehicle, sustain injury, or lose control of their vehicle entirely.  Similarly, sharp curves can result in motorists losing control, flipping their vehicles, and/or driving off the road and colliding with trees, telephone poles, or pedestrians.

To counteract these dangerous, a few safety devices can be put into place that warn motorists of upcoming curves or speed humps.  These can simply be signs that say, "Dangerous Curve Ahead", or "Reduce Speed", in addition to visible markers that outline the geometry of a curve for drivers.


The street being observed is a portion of Central Street, located along the Saugus Iron Works in Saugus, Massachusetts.  See below:

The arrows labeled A, B, C, and D represent the orientations of the observations made on the street

This portion of Central Street is highly residential, having several perpendicular two-way side streets that feed into it.  The road functions much like a collector.  Some traits of the street are listed below:

-30 mph speed limit
-3 ft. green space along sidewalks (serves as a safety buffer, but also has trees for shade, hydrants, telephone poles ect.)
-Sidewalk (varying from 4.5-6 ft. depending on which side and location along street) that is usually appropriate for two pedestrians to walk together
-Lane widths approximately 11-12 ft. wide

Taken from orientation A

Traffic Calming:

On majority of Central Street (which stretches a sizeable length across Saugus), motorists travel around 35-40 mph.  This may be attributed to the wider widths of the lanes and the lack of traffic calming measures.  On this portion of the road however, drivers are typically slowed to about 15 mph at the curves, and 20-25 mph in between the curves.

The images and clips below show the geometry and spacing of the three curves.  Due to their tight radii, drivers are forced to significantly reduce their speed (~15 mph) in order to safely execute the turn.  A motorist travelling at a speed of 25 mph or more has a high chance of losing control over his/her vehicle along the curve.  Additionally, the close spacing of the three curves to eachother influences drivers to keep their speed down (~25 mph between curves), much like the utilization of multiple speed humps/bumps within reasonable proximity of one another would.

Taken from orientation A

Taken from orientation B

Taken from orientation C

Also taken from orientation C

This clip shows some examples of motorists slowing down to safely handle the curves:

This clip shows the first-person view of a Northbound driver handling the curves:

At approach A, the driver's speed was about 35 mph.  Leading up to the first curve, it dropped down to 20, and then 15 mph to actually handle the curve.  Between B and C, the driver sped up to about 26 mph, and then reduced speed back to 15 mph.  Between C and D, the driver only managed to speed up to about 22 mph before taking the final curve.  After the last curve, the driver was able to increase his speed back up to 35 mph.

Further Traffic Calming Recommendations:

A few added treatments could enforce and improve the traffic calming features of this street:

Reduce the width of lanes:  The lanes on the road could be uniformly reduced down to about 10 ft.  To do this, a median strip could be added in the middle of the road (~3-4 ft. wide).  This would achieve a few things:  Improve pedestrian crossing safety, effectively reduce lane width, and influence drivers to stay in their lane while handling the curves, thus reinforcing the need to reduce their speed.

Utilize speed humps between curves A and C:  On average, drivers kept their speed down to about 25-28 mph between A and C.  The stretch between A and C is slightly longer than that of C and D, where drivers averaged around 20 mph.  By adding 2-3 light speed humps, designed for 20 mph speed, motorists' speed could be kept relatively uniform throughout this whole stretch of road.

Safety Measures:

There were some useful safety devices that were utilized to warn unsuspecting motorists who may be driving at night, or have never used this portion of Central Street before.  These signs help reduce dangerous situations by alerting motorists who are approaching the curves, and allowing them to prepare for the speed change.  See below:

Both pictures were taken from orientation A.  They are both located about 200 ft before the first curve and warn the driver that the curve is ahead. 
Taken from orientation A.  These markers are used at the first curve between A and B, and also at the curve at D.  They outline the edge of the curve and assist drivers who are approaching or taking the turn to judge the geometry of the curve

1 comment:

  1. Wow! There are way too many close calls and tragedies that could be prevented by drivers doing the right thing. Thanks for sharing your post!!
    Speed humps