The earliest days of the Capital Beltway

[agentsteel53]

There is a lot more to highway design standards than horizontal and vertical curvature.  There is lane width, median width, shoulder width, and clear recovery zone width.  The NJTP median was only 25 feet wide when built, and had no median barrier.  It was built to no higher design standards than later rural Interstate highways, actually less.

those are safety standards, not physical ones.  a car can do 80 on a road with subpar medians and narrow lanes, even though - from a modern safety perspective - it should not.  the same car physically cannot do 80 around a curve without adequate radius and superelevation.

[Alps]

And I was assuming 12-foot lanes and nonzero shoulders. He is right in that narrower lanes and shoulders can limit design speed. However, I haven't seen a study that says if 12-foot lanes are appropriate for 60 mph, should you increase for 85 mph?

[Beltway]

And I was assuming 12-foot lanes and nonzero shoulders. He is right in that narrower lanes and shoulders can limit design speed. However, I haven't seen a study that says if 12-foot lanes are appropriate for 60 mph, should you increase for 85 mph?

I haven't seen a study that says if a freeway is completely tangent and level for 20 miles, should the design speed be 120+ mph?

[agentsteel53]

I haven't seen a study that says if a freeway is completely tangent and level for 20 miles, should the design speed be 120+ mph?

depends on surface quality. 

what is the design speed of a typical no-speed-limit German autobahn segment, and how does it compare to the 130 km/h recommended speed?

[Revive 755]

And I was assuming 12-foot lanes and nonzero shoulders. He is right in that narrower lanes and shoulders can limit design speed. However, I haven't seen a study that says if 12-foot lanes are appropriate for 60 mph, should you increase for 85 mph?

IIRC, one of the studies for a Chicago-KC tollway with higher than usual posted speeds recommended lanes widths of 12.5' for 85 mph and 13' for 100 mph.

[Beltway]

And I was assuming 12-foot lanes and nonzero shoulders. He is right in that narrower lanes and shoulders can limit design speed. However, I haven't seen a study that says if 12-foot lanes are appropriate for 60 mph, should you increase for 85 mph?

IIRC, one of the studies for a Chicago-KC tollway with higher than usual posted speeds recommended lanes widths of 12.5' for 85 mph and 13' for 100 mph.

I would recommend for 80 mph, 13-foot-wide lanes, 12-foot-wide right and left shoulders, and 30 feet width of clear zone on each side of the directional roadways.

[deathtopumpkins]

What does Texas use for their 80 mph [and possibly soon 85 mph] zones? I'm willing to bet they built those roads to regular interstate standards, including 12 ft lanes.

[agentsteel53]

What does Texas use for their 80 mph [and possibly soon 85 mph] zones? I'm willing to bet they built those roads to regular interstate standards, including 12 ft lanes.

85 is only going to be applicable to certain roads approved after Sept 2011 (in this case, I believe only the 130 toll road).  I do not know what the standards are, but I seem to recall reading that they are an upgrade over the interstate standards.

that said, I believe some sections of I-10 and I-20 with a speed limit of 80 were opened as early as 1957-1959!