diff --git a/src/propagation/model/propagation-delay-model.cc b/src/propagation/model/propagation-delay-model.cc
index e920b6a57..aea0dcc6f 100644
--- a/src/propagation/model/propagation-delay-model.cc
+++ b/src/propagation/model/propagation-delay-model.cc
@@ -92,8 +92,8 @@ ConstantSpeedPropagationDelayModel::GetTypeId (void)
   static TypeId tid = TypeId ("ns3::ConstantSpeedPropagationDelayModel")
     .SetParent<PropagationDelayModel> ()
     .AddConstructor<ConstantSpeedPropagationDelayModel> ()
-    .AddAttribute ("Speed", "The speed (m/s)",
-                   DoubleValue (300000000.0),
+    .AddAttribute ("Speed", "The propagation speed (m/s) in the propagation medium being considered. The default value is the propagation speed of light in the vacuum.",
+                   DoubleValue (299792458),
                    MakeDoubleAccessor (&ConstantSpeedPropagationDelayModel::m_speed),
                    MakeDoubleChecker<double> ())
   ;