{"id":130,"date":"2026-08-23T09:26:36","date_gmt":"2026-08-23T01:26:36","guid":{"rendered":"https:\/\/www.livablehub.com\/?p=130"},"modified":"2026-08-23T09:26:37","modified_gmt":"2026-08-23T01:26:37","slug":"only-5-of-congestion-is-caused-by-poor-signal-timing-is-this-20-year-old-figure-still-valid","status":"publish","type":"post","link":"https:\/\/www.livablehub.com\/?p=130","title":{"rendered":"\u201cOnly 5% of Congestion Is Caused by Poor Signal Timing\u201d \u2014 Is This 20-Year-Old Figure Still Valid?"},"content":{"rendered":"\n
\"\"<\/figure>\n\n\n\n

Reference: Traffic Congestion and Reliability: Linking Solutions to Problems<\/em>, Federal Highway Administration (FHWA), July 19, 2004. Report No. FHWA-HOP-05-004<\/p>\n\n\n\n

A widely circulated FHWA chart breaks down the causes of traffic congestion in the United States. One number in particular often attracts attention:<\/p>\n\n\n\n

\n

Only 5% of congestion is caused by poor signal timing.<\/strong><\/p>\n<\/blockquote>\n\n\n\n

But this chart is now more than 20 years old. So the obvious question is: is it still 5% today?<\/strong><\/p>\n\n\n\n

1. Where Did the 5% Figure Come From?<\/h2>\n\n\n\n

The figure comes from a 2004 report published by the Federal Highway Administration (FHWA)<\/strong> titled Traffic Congestion and Reliability: Linking Solutions to Problems<\/em>.<\/p>\n\n\n\n

The report estimated the sources of congestion as follows:<\/p>\n\n\n\n

    \n
  • 40% \u2014 Bottlenecks<\/strong><\/li>\n\n\n\n
  • 25% \u2014 Traffic incidents<\/strong><\/li>\n\n\n\n
  • 15% \u2014 Bad weather<\/strong><\/li>\n\n\n\n
  • 10% \u2014 Work zones<\/strong><\/li>\n\n\n\n
  • 5% \u2014 Poor signal timing<\/strong><\/li>\n\n\n\n
  • 5% \u2014 Special events and other causes<\/strong><\/li>\n<\/ul>\n\n\n\n

    There is, however, an important detail.<\/p>\n\n\n\n

    This was not a nationwide census of every congested road in the United States<\/strong>.<\/p>\n\n\n\n

    FHWA described the figures as a \u201ccomposite estimate,\u201d<\/strong> meaning that they were based on a combination of multiple existing studies. FHWA also explicitly referred to them as \u201crough approximations.\u201d<\/strong><\/p>\n\n\n\n

    The figures were later cited repeatedly in other FHWA publications. For example, the 2012 Urban Congestion Trends<\/em> report was still using the same 40%\u201325%\u201315%\u201310%\u20135%\u20135% breakdown.<\/p>\n\n\n\n

    In 2016, FHWA revisited the issue in Traffic Bottlenecks: Identification and Solutions<\/em>. The report noted that FHWA had been citing the \u201c40% bottlenecks + 5% poor signal timing\u201d figures for at least 15 years, and that the estimates were largely based on earlier studies, particularly work associated with the I-95 corridor.<\/p>\n\n\n\n

    So strictly speaking, the 5% figure is a historical estimate, not a number that has been recalculated every year.<\/strong><\/p>\n\n\n\n

    2. What Do the Latest Data Say?<\/h2>\n\n\n\n

    I looked at FHWA\u2019s latest publicly available national congestion report.<\/p>\n\n\n\n

    The most recent report is 2024 Urban Congestion Trends<\/strong>, published by FHWA in 2025. It covers traffic conditions across 52 large metropolitan areas in the United States<\/strong>.<\/p>\n\n\n\n

    \"\"<\/figure>\n\n\n\n

    Reference: FHWA, 2024 Urban Congestion Trends<\/em>, Report No. FHWA-HOP-25-195<\/p>\n\n\n\n

    The 2024 results show that:<\/p>\n\n\n\n

      \n
    • Average daily congested time increased by 38 minutes<\/strong> compared with 2023.<\/li>\n\n\n\n
    • That represents an increase of more than 20%<\/strong>.<\/li>\n\n\n\n
    • In 92% of the 52 metropolitan areas<\/strong>, all three major congestion indicators became worse.<\/li>\n<\/ul>\n\n\n\n

      What FHWA measures today, however, is somewhat different.<\/p>\n\n\n\n

      The current focus is mainly on indicators such as:<\/p>\n\n\n\n

        \n
      • Congested Hours<\/strong><\/li>\n\n\n\n
      • Travel Time Index (TTI)<\/strong><\/li>\n\n\n\n
      • Planning Time Index (PTI)<\/strong><\/li>\n<\/ul>\n\n\n\n

        In other words, FHWA now puts more emphasis on questions such as:<\/p>\n\n\n\n

          \n
        • How many hours per day is a corridor congested?<\/li>\n\n\n\n
        • How much longer does a trip take compared with free-flow conditions?<\/li>\n\n\n\n
        • How reliable or unpredictable is travel time?<\/li>\n<\/ul>\n\n\n\n

          The latest report does not provide a new national breakdown of congestion causes<\/strong>.<\/p>\n\n\n\n

          So there is currently no updated FHWA figure that allows us to say:<\/p>\n\n\n\n

          \n

          \u201cPoor signal timing still accounts for exactly 5% of congestion today.\u201d<\/p>\n<\/blockquote>\n\n\n\n

          3. Why Do We Rarely See National Breakdowns Like 40%, 25% and 5% Anymore?<\/h2>\n\n\n\n

          One reason is that congestion is difficult to attribute to a single cause<\/strong>.<\/p>\n\n\n\n

          Imagine a freeway that is already operating close to capacity. Rain begins, which reduces traffic speed. Then a minor crash blocks one lane, and the resulting queue rapidly grows. How should that congestion be classified? Weather? An incident? Or insufficient road capacity? In reality, these factors often interact.<\/p>\n\n\n\n

          Producing a national \u201ccause of congestion\u201d percentage also requires researchers to match travel-time data with detailed information about incidents, construction, weather, special events, signal operations and other conditions.<\/p>\n\n\n\n

          Transportation agencies may have very good speed and travel-time data, but they do not always have a complete cause label<\/strong> for every period of congestion.<\/p>\n\n\n\n

          The United States has also gradually shifted toward measuring outcomes rather than assigning every delay to a specific cause<\/strong>.<\/p>\n\n\n\n

          Since around 2012, FHWA has placed greater emphasis on measures such as travel-time reliability, freight reliability and peak-hour delay.<\/p>\n\n\n\n

          National datasets such as NPMRDS<\/strong> can continuously tell agencies:<\/p>\n\n\n\n

          \n

          Is this road congested?
          How long is it congested?
          Is it worse than last year?<\/p>\n<\/blockquote>\n\n\n\n

          But speed and travel-time data alone cannot always answer a different question:<\/p>\n\n\n\n

          \n

          Why exactly did this congestion happen?<\/strong><\/p>\n<\/blockquote>\n\n\n\n

          So, for now, the answer is fairly simple:<\/p>\n\n\n\n

          There is no new FHWA national percentage that replaces the 2004 \u201c5%\u201d figure.<\/strong><\/p>\n\n\n\n

          The number remains a useful historical reference, but it should not be presented as a current nationwide statistic.<\/p>\n","protected":false},"excerpt":{"rendered":"

          In 2004, the U.S. Federal Highway Administration (FHWA) estimated that only about 5% of traffic congestion was caused by poor signal timing. That figure was widely cited for years, but more than two decades later, FHWA has not published a new national breakdown of congestion causes.<\/p>\n","protected":false},"author":1,"featured_media":131,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[26],"tags":[35,24],"class_list":["post-130","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-traffic-signal","tag-fhwa","tag-traffic-signal"],"jetpack_featured_media_url":"https:\/\/www.livablehub.com\/wp-content\/uploads\/2026\/08\/ScreenShot_2026-08-23_092224_802.png","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/www.livablehub.com\/index.php?rest_route=\/wp\/v2\/posts\/130","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.livablehub.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.livablehub.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.livablehub.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.livablehub.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=130"}],"version-history":[{"count":1,"href":"https:\/\/www.livablehub.com\/index.php?rest_route=\/wp\/v2\/posts\/130\/revisions"}],"predecessor-version":[{"id":133,"href":"https:\/\/www.livablehub.com\/index.php?rest_route=\/wp\/v2\/posts\/130\/revisions\/133"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.livablehub.com\/index.php?rest_route=\/wp\/v2\/media\/131"}],"wp:attachment":[{"href":"https:\/\/www.livablehub.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=130"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.livablehub.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=130"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.livablehub.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=130"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}