<story><title>Why are Texas interchanges so tall?</title><url>https://practical.engineering/blog/2024/8/19/why-are-texas-interchanges-texas-so-tall</url></story><parent_chain><item><author>mikepurvis</author><text>From a quick skim of Wikipedia, it looks like the main benefit to the stack style over a conventional 2-level cloverleaf is capacity (the left-turn leaves are generally at most one lane) and speed (left turns are direct rather than 270deg, so you don&amp;#x27;t have to slow down as much when taking them).&lt;p&gt;Still, to this layman it seems like a colossal engineering cost to build all those bridges and ramps for relatively modest gain.</text></item><item><author>missingcolours</author><text>There are frontage roads at most freeway interchanges in Texas.&lt;p&gt;Typical stack interchange elsewhere has 4 levels: freeway 1, freeway 2, freeway 1 left turns, freeway 2 left turns. Frontage roads add 5th and occasionally 6th levels.&lt;p&gt;The other thing not mentioned is the proliferation of separate express lanes, which often have dedicated flyover ramps as well.</text></item><item><author>Groxx</author><text>So...&lt;p&gt;&lt;pre&gt;&lt;code&gt;    &amp;gt; Q: Why are Texas interchanges so tall?
    &amp;gt;
    &amp;gt; Frontage roads need grade separation, adding a layer.
    &amp;gt; Texas has lots of frontage roads.
&lt;/code&gt;&lt;/pre&gt;
OK, I buy that Texas has &lt;i&gt;more&lt;/i&gt; grade-separated interchanges on average... but I&amp;#x27;m not seeing how that turns into &amp;quot;Texas has more higher interchanges&amp;quot; which is what it feels like that question is asking.  Frontage roads don&amp;#x27;t generally exist where 3+ highways intersect, so that shouldn&amp;#x27;t be relevant for high ones, it just raises many 1-high to 2-high.&lt;p&gt;Or do they, in Texas?  Or is it just that Texas has one famous one and people extrapolate that to mean the whole state is absolutely smothered in interchanges, until it looks like a scene from The Fifth Element?</text></item></parent_chain><comment><author>toast0</author><text>Cloverleaf intersections have really poor overload behavior. They&amp;#x27;re fine in low traffic areas, but in places with high traffic, the crossing streams of traffic cause cascading backups.&lt;p&gt;Stack interchanges increase acheivable throughput because you can often build enough road to allow backups for one direction to not impinge on the other flows.&lt;p&gt;It&amp;#x27;s also easier to build effective signage so there&amp;#x27;s fewer surprise lane changes.</text></comment>
<story><title>Why are Texas interchanges so tall?</title><url>https://practical.engineering/blog/2024/8/19/why-are-texas-interchanges-texas-so-tall</url></story><parent_chain><item><author>mikepurvis</author><text>From a quick skim of Wikipedia, it looks like the main benefit to the stack style over a conventional 2-level cloverleaf is capacity (the left-turn leaves are generally at most one lane) and speed (left turns are direct rather than 270deg, so you don&amp;#x27;t have to slow down as much when taking them).&lt;p&gt;Still, to this layman it seems like a colossal engineering cost to build all those bridges and ramps for relatively modest gain.</text></item><item><author>missingcolours</author><text>There are frontage roads at most freeway interchanges in Texas.&lt;p&gt;Typical stack interchange elsewhere has 4 levels: freeway 1, freeway 2, freeway 1 left turns, freeway 2 left turns. Frontage roads add 5th and occasionally 6th levels.&lt;p&gt;The other thing not mentioned is the proliferation of separate express lanes, which often have dedicated flyover ramps as well.</text></item><item><author>Groxx</author><text>So...&lt;p&gt;&lt;pre&gt;&lt;code&gt;    &amp;gt; Q: Why are Texas interchanges so tall?
    &amp;gt;
    &amp;gt; Frontage roads need grade separation, adding a layer.
    &amp;gt; Texas has lots of frontage roads.
&lt;/code&gt;&lt;/pre&gt;
OK, I buy that Texas has &lt;i&gt;more&lt;/i&gt; grade-separated interchanges on average... but I&amp;#x27;m not seeing how that turns into &amp;quot;Texas has more higher interchanges&amp;quot; which is what it feels like that question is asking.  Frontage roads don&amp;#x27;t generally exist where 3+ highways intersect, so that shouldn&amp;#x27;t be relevant for high ones, it just raises many 1-high to 2-high.&lt;p&gt;Or do they, in Texas?  Or is it just that Texas has one famous one and people extrapolate that to mean the whole state is absolutely smothered in interchanges, until it looks like a scene from The Fifth Element?</text></item></parent_chain><comment><author>seijiotsu</author><text>Don’t forget ease and safety. With a stack interchange (and many other types), you often have plenty of time beforehand to position yourself into the correct lane. But if you are going to go through a cloverleaf you will often need to make a stressful weave into the leaf, and then back again into the highway.</text></comment>