The 1997 & 2000 HCM methodologies for analysis of unsignalized intersections changed significantly from the methodology used in the 1994 HCM. The following is a summary of the major changes.
An entirely new capacity formula was provided that takes into account effects of upstream signals, pedestrians, flared approaches, and median refuges.
A formula was provided for estimating queue lengths.
The delay formula was modified and a new formula for major street delay was provided.
New LOS thresholds consistent with the new formulae were established (see the tabulation below).
|
1997 & 2000 HCM Unsignalized LOS Delay Thresholds |
|
|
Level of Service |
|
|
A |
< 10 |
|
B |
> 10 and < 15 |
|
C |
> 15 and < 25 |
|
D |
> 25 and < 35 |
|
E |
> 35 and < 50 |
|
F |
> 50 |
Table 14 1997 & 2000 HCM Unsignalized LOS Delay Thresholds
Users should consult the 1997 and 2000 Highway Capacity Manual for details of these changes.
The user can enter the analysis time period in hours (default=0.25) on the Geometry... LOS Method screen.
Same as 1994 HCM method, using on the Geometry LOS Method screen.
Similar to the 1994 HCM method, using the Geometry PCE screen.
The 1997 & 2000 HCM methods determine critical gap on the basis of the percentage of heavy vehicles
and the roadway grades. These values can be entered on the Geometry Critical Gap screen.
There are several new parameters that need to be entered for the 1997 & 2000 HCM methods. These are entered on Geometry ... LOS Method screen, and include Flared Lane Queue, Pedestrian Volumes, Lane Width, Gap Storage, Pedestrian Walk Speed, and Median Type.
Additional data need to be entered to account for the platoon effects of upstream signalized intersections within 0.25 miles of the subject intersection. These include the following data for the upstream intersection:
Cycle length (sec) – entered at the upstream signal node;
Effective green time (sec) for the major-street through movement and, if applicable, for the exclusive left-turn phase front the minor street – entered at the upstream signal node;
Lanes and Saturation flow rate in vehicles per hour of green (veh/hrg) – entered at the upstream signal node;
Distance from the signalized intersection to the subject TWSC intersection – entered on the link(s) approaching the unsignalized intersection from the upstream signal(s);
Speed of the platoon as it progresses from the signalized intersection to the TWSC intersection – entered on the link(s) approaching the unsignalized intersection from the upstream signal (s);
Through flow rate arriving at the signalized intersection on the major-street approach and, if applicable and significant, (the left-turn flow rate from the side street during an exclusive left turn phase – entered at the upstream signal node; and
Arrival type of vehicles at the signalized intersection – entered at the upstream signal node.
To enter these data items, first create adjacent upstream intersections and links connecting them to the subject unsignalized intersection. Enter the speed and distance data in the link data dialog boxes for the links approaching the unsignalized intersection. Enter the other data items in the Volume and Geometry dialog boxes for the upstream signalized intersections.
If the user is analyzing only the unsignalized intersection, the data entry at the upstream intersections is limited to the items listed above. If the user is analyzing the upstream intersections as well, TRAFFIX™ will automatically pull the needed data from those intersections. Note, however, that unless data for an arterial route analysis has been entered, the link data (speed and distance) will not ordinarily be present and the user will still have to enter those data items on the links.