Assumption 5. We assume that vehicles are separated, on average,
by a follow-up headway of "hw" seconds. We specify
the headway in terms of the time interval between vehicles at whatever
speed they are traveling. Headway (in seconds) is equal to the separation
distance (in feet) divided by the speed (in feet/sec). Here are
a few examples at various speeds.
Headway is a real physical quantity that can be measured and studied
under various conditions. There are some general characteristics
of headway that are model-independent. Traffic conditions with lower
headway are perceived by drivers as higher in congestion and are
generally less safe.
As an ingredient in a capacity model, follow-up headway (hw)
can be considered a parameter that idealizes the very complicated
reality of vehicle capabilities, roadway conditions, and driver
Examples of some important vehicle characteristics include
- max acceleration and deceleration
- turning agility
- control interface with driver
- current state (maintenance, operating conditions, breakdowns)
Roadway characteristics that impact safe and achievable headways
- curvature and inclination
- lines of sight
- current state (maintenance, accidents, weather, traffic)
Driver behaviors that affect safe and achievable headways include
- mental and physical capabilities
- training and experience
- current state (alertness, attentiveness, fatigue, emotions,
In a sense, vehicles and roadways are designed to give a usable
set of transportation tools, and "users" (mainly automobile
drivers, in the current world of NM502) are the agents who use the
tools to transport people and goods according to their best real-time
So, in the context of our simple model for roadway capacity, a
single value of hw can be viewed as an average over a set
of drivers and vehicles under a particular kind of driving conditions.
We could use a range of hw values to consider different
kinds of drivers, roadways, or driving conditions.
There are several ways one could motivate a particular follow-up
headway value or range of values.
(1) One could use a guideline specified in the NM Driver's Manual
called the "3-second rule" (described here,
p.32; note that this guidance, while useful, is not conservative
enough to avoid multi-car collisions in the case of an instantaneous
(2) One could use a US-wide average from a standards document,
such as the 2010 Highway Capacity Manual and/or a value from the
roadway modeling literature, for example Akcelik
(International Roundabout Conference, Transportation Research Board,
Carmel, IN, May 18-20, 2011). The average headway recommended here
is 3.2 sec.
(3) One could use a local measurement. Los Alamos County traffic
data collected at two points along NM502 on August 12, 2010 yielded
average headway at peak times of 2.8 sec (with a standard deviation
of 0.14 sec).
Since these are fairly consistent, we will choose a "nominal"
value of hw = 3 sec. To consider a variety of vehicle,
roadway, and driver characteristics, we'll look at a range of headway
values between 2 sec (straight roads, aggressive drivers, optimum
weather and roadway conditions) and 4 sec (sharp curves, defensive
drivers, adverse weather conditions).